Chapter IV: , pp. 21-36. The Herbert-Smith refractometer is there (4)

RUBIES. Passing on to the occurrence of the _corundum_ gems we will consider first the _ruby_. Most fine rubies come from Burmah. The district in which they are found is near Mogok. Practically all the fine pigeon-blood rubies come from this district. The fashion for red stones being for the time little in evidence rubies are not now in great demand. This cessation of demand can hardly be laid to the competition of the scientific ruby, for the sapphire is now very much in vogue, yet scientific sapphires resemble the natural ones even more closely than do the rubies.

Siam furnishes a considerable number of dark garnet-like rubies. These do not command high prices. They are, however, sometimes very beautiful, especially when well cut for brilliancy, and when in a strong light.

Ceylon furnishes a few rubies and a few red corundums have been found in North Carolina.

The Burmese rubies appear to have been formed in a limestone matrix, but most of those obtained are gotten from the stream beds, where they have been carried by water after weathering out from the mother rock.

The rubies of Ceylon, too, probably originated in a limestone matrix, but are sought in stream gravels.

SAPPHIRES. Fine blue sapphires originate in Siam in larger numbers than in any other locality. Kashmir, in India, also supplies splendid specimens of large size. Ceylon, too, furnishes a good deal of sapphire, but mostly of a lighter color than the Kashmir sapphire. The Ceylon sapphires are found in the streams, but originate in rock of igneous origin.

Montana furnishes considerable quantities of sapphire, some of which is of very good color. It is, of course, as good as the Oriental if of equal color, being of the same material. The better colored sapphire from Montana is mined from the rock. Most of the sapphires found in the river gravels near Helena, Mont., are greenish blue or of other colors, and not of fine blue.

Queensland and Victoria in Australia supply considerable quantities of sapphire. When blue the Australian sapphire is usually too dark to be very valuable. The golden and other "fancy" sapphires of the trade come largely from the Ceylon gravels. Siam yields silky brown stones and some fine green ones. Some of the Australian sapphires when cut in certain directions yield green stones.

CHRYSOBERYL. Chrysoberyl of the variety Alexandrite now comes mainly from Ceylon, although formerly from the Ural Mountains.

The cat's-eyes also come chiefly from Ceylon.

The yellowish-green chrysoberyls (which jewelers sometimes call chrysolite) come both from Ceylon and from Brazil. They are frequently found in papers of "fancy sapphires" or "fancy color stones," so called.

SPINEL. Spinels are found along with ruby in Burmah and in Siam and they also occur in the gem gravels of Ceylon. Limestone is the usual matrix of spinel, although it is more often mined in gravels resulting from the weathering of the matrix.

TOPAZ. True topaz, of wine-yellow color, comes mostly from Brazil. Ceylon also furnishes yellow topaz. Asiatic Russia furnishes fine large blue or blue-green crystals resembling aquamarine in appearance. Most of the topaz found in other localities is pale or colorless. Several of our western States, notably Utah, Colorado, and California, furnish colorless topaz. Mexico and Japan also produce it. It is seldom cut, for, while producing a rather brilliant stone, it has little "fire" and is therefore not very attractive.

EMERALD AND AQUAMARINE. Beryl of the emerald variety is exceedingly scarce in the earth. Most of the best emerald comes from Colombia, South America. Large crystals of paler color come from the Urals.

Like ruby and spinel, emerald usually originates in limestone. One is tempted to suspect that these stones are of aqueous origin and that sapphires, and beryl, other than emerald, are more likely of igneous origin.

Beryls of the aquamarine type occur in many places, but usually of too pale a tint, or too imperfect, to be worthy of cutting. Fine gem beryl of blue and blue-green tints comes from Siberia and from several places in the Ural Mountains on their Asiatic slopes.

The Minas Geraes district of Brazil, famous for all kinds of gem stones, furnishes most of the aquamarine of commerce. The pegmatite dikes of Haddam Neck, Conn., of Stoneham, Me., and of San Diego County, Cal., have furnished splendid aquamarine and other beryl. These dikes, according to the geological evidence, are the result of the combined action of heat and water. Thus both melting and dissolving went on together and as a result many fine gem minerals of magnificent crystallization were formed during the subsequent cooling. The longer the cooling lasted and the more free space for growth the crystals had, the larger and more perfect they got. The author has himself obtained finely crystallized aquamarine and tourmaline from the Haddam, Conn., locality and the best specimens there occur in "pockets" or cavities in the coarse granite. Within, these pockets are lined with crystals of smoky quartz, tourmaline, beryl, and other minerals. Sometimes crystals occur in mud or clay masses inside the cavities and such crystals, having been free to grow uninterruptedly in every direction, were perfect in form, being doubly terminated, and not attached anywhere to the rock.

Madagascar has in recent years furnished the finest pink beryl, which has been named Morganite. Yellow beryl (Heliodor) and aquamarine also occur in Madagascar.

ZIRCON. Zircon comes on the market mainly from Ceylon. It deserves to be as much esteemed in this country as it is in Ceylon, for its optical properties are such that it is a very snappy stone. Some of the colors in which it occurs, such as the golden browns, lend themselves nicely to the matching of gems and garments, and, with the growth of education in such matters, jewelers would do well to get better acquainted with the possibilities of zircon and to introduce it to their customers. The supply from Ceylon is sufficient to justify popularizing the stone. Small zircons are found in almost every heavy concentrate, as, for example, in the concentrates of the diamond mines of South Africa, and in those of gold placers in many places. The rough stones resemble rough diamonds in luster and are sometimes mistaken for diamonds.

GARNETS. Garnets of various types are found widely distributed in nature. Perhaps the Bohemian supply is best known, having furnished a host of small stones which have usually been rose cut for cluster work or made into beads. The Bohemian garnets are of the pyrope or fire-red type. Relatively few large stones of sufficient transparency for cutting are produced in the Bohemian mines. The so-called "Cape rubies" of the diamond mines of South Africa are pyrope garnets and some large and fine ones are found. The "Arizona rubies" are pyrope garnets, and while seldom of notable size, some are of very fine color, approaching deep rubies, and the color remains attractive by artificial light.

Almandite garnet, the "almandine" of the jeweler is less abundant than pyrope, when of gem quality. Ceylon furnishes some and India furnishes perhaps more. Brazil, from its prolific gem gravels at Minas Novas, supplies good almandite, and smaller quantities are found in many different localities.

Hessonite garnet, the cinnamon stone or "hyacinth" (incorrect) of the trade, comes mainly from Ceylon.

Andradite garnet, of the variety known as demantoid, from its diamond-like properties, and which is usually sold under the misleading name "olivine" in the trade, comes from the western slopes of the Ural Mountains.

TOURMALINE. Gem tourmaline comes from Ceylon, from Madagascar, from the Ural Mountains, from Brazil, from Maine, from Connecticut, and from California.

The Ceylon tourmalines are mostly yellow or yellowish green, sometimes fine olive-green. Those from the Urals may be pink, blue or green. Brazilian tourmalines are usually green, but sometimes red. In fact in many localities several colors of tourmaline are usually found together and it may be that a single crystal will be green in most of its length but red or pink tipped. Some crystals have a pink core and a green exterior. The author has found both of the two latter types in the Haddam, Conn., tourmalines, and on one occasion was surprised to get back a wine-colored tourmaline from a cutter to whom he had sent a green crystal. There was but a thin shell of the green material on the outside of the crystal.

Some of the Madagascar tourmaline is of a fine brownish red, almost as deep as a light garnet, and much clearer than most garnet.

Would it not be fitting on account of its occurrence in several localities in the United States, for Americans to use more tourmaline in their jewels? The quality of some of the tourmalines of Maine, and of California especially, is not excelled by tourmaline from any other locality. Some of the Maine tourmaline is of a delightful, slightly bluish-green tint that almost approaches emerald.

KUNZITE. Spodumene, of the variety kunzite, comes from San Diego County, California.

QUARTZ GEMS. Coming now to the quartz gems we find amethyst and citrine, or golden quartz widely distributed so that only the localities that furnish the better grades of these stones need be mentioned. Siberia and Uruguay furnish fine amethyst. Brazil also furnishes large quantities of very good quality.

AMETHYST. The chief charm of the Siberian amethyst lies in its large red component, which enables it to change from a deep grape-purple by daylight to a fine red by artificial light that is rich in red rays, and poor in blue ones. The paler types of amethysts that were once esteemed, probably for lack of the rich deep variety, become gray in appearance and much less lovely under artificial light. India furnishes some amethysts, and papers of "fancy color stones" containing native cut gems from Ceylon, frequently contain amethysts, but Brazil, Uruguay, and Siberia furnish the great bulk of the stones that are regarded as choice to-day.

YELLOW QUARTZ. Citrine or golden quartz comes mainly from Brazil. The "Spanish topaz" is sometimes the result of heating smoky quartz from Cordova province in Spain. Our own western mountains furnish considerable yellow and smoky quartz fit for cutting.

ROSE QUARTZ. Rose quartz of the finest quality comes from South Dakota. Bavaria, the Ural Mountains, and Paris, Maine, have also furnished it.

AGATE. Agates of the finest types, such as carnelian and sard, come principally from Brazil and from India.

OPAL. Opals now come most largely from Australia, the Hungarian mines yielding but few stones at present. The fine black opals of New South Wales are unsurpassed by any that have ever been found elsewhere. Mexico furnishes considerable opal, and is notable for its fine "fire opal" or "cherry opal."

JADE. Most of the jade of the variety nephrite that is obtained to-day comes from several of the provinces of China or from Siberia or from Turkestan. A dark-green nephrite comes from New Zealand.

Jade of the jadeite variety, which is harder than nephrite and more highly valued, is rare. The best specimens come from Upper Burmah. It is also found in China and in Tibet.

PERIDOT. Peridot, and the brighter olivine or chrysolite, while of the same mineral species, do not seem to occur together. The darker bottle-green specimens come from the Island of St. John in the Red Sea. It is said that many of the finer peridots now available have been recut from old stones mined many years ago.

Queensland supplies light-green chrysolite, and Arizona a yellowish-green variety. Light-green stones have been found near the ruby mines of Upper Burmah.

MOONSTONE. Moonstone comes mainly from Ceylon. The native cut specimens are sent here and recut, as, when native cut, the direction of the grain is seldom correct to produce the moonlight effect in symmetrical fashion. The native cutters apparently try to retain all the size and weight that is possible, regardless of the effect.

TURQUOISE. Turquoise of the finest blue and most compact texture (and hence least subject to color change) comes from the province of Khorasan in Persia. Several of our western states supply turquoise of fair quality, notably New Mexico, Arizona, Nevada, and California.

LAPIS LAZULI. Lapis Lazuli comes from Afghanistan, from Siberia, and from South America.

MALACHITE. Malachite is found in many copper mines, but principally in those of the Ural Mountains.

AZURITE. Azurite is found in the Arizona mines and in Chessy, in France (hence the name chessylite, sometimes used instead of azurite).

* * * * *

REFERENCES. Students who wish to get a fuller account of the occurrence of precious stones should run through G. F. Herbert-Smith's _Gem-Stones_ under the different varieties. This work is the most recent authentic work of a strictly scientific character. Dr. George F. Kunz's _Gems and Precious Stones of North America_ gives a detailed account of all the finds in North America up to the time of publication. Many of these are of course of little commercial importance. The _Mineral Resources of the United States_ contains annually a long account of the occurrences of gem materials in this country. A separate pamphlet containing only the gem portion can be had gratis from the office of the United States Geological Survey, Washington, D. C.

LESSON XXII

HOW ROUGH PRECIOUS STONES ARE CUT

ROUGH PRECIOUS STONES. John Ruskin, who had the means to acquire some very fine natural specimens of gem material was of the opinion that man ought not to tamper with the wonderful crystals of nature, but that rather they should be admired in the rough. While one can understand Ruskin's viewpoint, nevertheless the art of man can make use of the optical properties of transparent minerals, properties no less wonderful than those exhibited in crystallization, and indeed intimately associated with the latter, and, by shaping the rough material in accordance with these optical properties, greatly enhance the beauty of the gem.

No material illustrates the wonderful improvement that may be brought about by cutting and polishing better than diamond. In the rough the diamond is less attractive in appearance than rock crystal. G. F. Herbert-Smith likens its appearance to that of soda crystals. Another author likens it to gum arabic. The surface of the rough diamond is usually ridged by the overlapping of minute layers or strata of the material so that one cannot look into the clear interior any more than one can look into a bank, through the prism-glass windows that are so much used to diffuse the light that enters by means of them. Being thus of a rough exterior the uncut diamond shows none of the snap and fire which are developed by proper cutting.

As the diamond perhaps shows more improvement on being cut than any other stone, and as the art of cutting the diamond is distinct from that of cutting other precious stones, both in the method of cutting and in the fact that the workers who cut diamonds cut no other precious stones, it will be well to consider diamond cutting separately.

Before discussing the methods by which the shaping and polishing are accomplished let us consider briefly the object that is in view in thus altering the shape and smoothing the surface of the rough material.

HOW CUTTING INCREASES BRILLIANCY. Primarily the object of cutting a diamond is to make it more brilliant. So true is this that the usual form to which diamonds are cut has come to be called the _brilliant_. The adjective has become a noun. The increased brilliancy is due mainly to two effects: First, greatly increased reflection of light, and second, dispersion of light. The reflection is partly external but principally internal.

Taking up first the internal reflection which is responsible for most of the white brilliancy of the cut stone we must note that it is a fact that light that is passing through any transparent material will, upon arriving at any polished surface, either penetrate and emerge or else it will be reflected within the material, depending upon the angle at which the light strikes the surface. For each material there is a definite angle outside of which light that is passing as above described, is _totally reflected_ within the material.

_AB_ represents the back surface of a piece of diamond.

_CD_ is a line perpendicular to _AB_.

Angle _CDE_ is about 24 degrees.

Dotted line, _FDH_ represents the course taken by a ray of light which is totally reflected at _D_ in such fashion that angle _FDA_ equals angle _HDB_.

Any light proceeding towards _AB_ but between _E_ and _C_, would fail to be totally reflected. Most of it would penetrate _AB_.]

TOTAL REFLECTION. For diamond this _critical angle_, as it is called, is very nearly 24° from a perpendicular to the surface. If now, we shape a diamond so that most of the light that enters it from the front falls upon the first back surface that it meets, at an angle greater than 24° to a perpendicular to that surface, the light will be totally reflected within the stone. The angle at which it is reflected will be the same as that at which it meets the surface. In other words the angles of incidence and of reflection are equal. See Fig. 9 for an illustration of this point.

THEORY OF THE "BRILLIANT." In the usual "brilliant" much of the light that enters through the front surface is thus totally reflected from the first rear facet that it meets and then proceeds across the stone to be again totally reflected from the opposite side of the brilliant. This time the light proceeds toward the top of the stone. See Fig. 10--(From G. F. Herbert-Smith's _Gem-Stones_).

The angles of the top of a brilliant are purposely made so flat that the up coming light fails to be totally reflected again and is allowed to emerge to dazzle the beholder. In the better made brilliants the angle that the back slope makes with the plane of the girdle is very nearly 41° and the top angle, or angle of the front slope to the plane of the girdle is about 35°. Such well made brilliants when held up to a bright light appear almost black--that is, they fail to pass any of the light through them (except through the tiny culet, which, being parallel to the table above, passes light that comes straight down to it).

In other words, instead of allowing the light to penetrate them, well-made brilliants almost totally reflect it back toward its source, that is, toward the front of the stone. The well-cut diamond is a very brilliant object, viewed from the front.

We must now consider how the "fire" or prismatic color play is produced, for it is even more upon the display of fire than upon its pure white brilliancy that the beauty of a diamond depends.

CAUSE OF "FIRE." As we saw in Lesson X. (which it would be well to re-read at this time), white light that changes its course from one transparent medium to another at any but a right angle to the surface involved, is not only refracted (as we saw in Lesson II.) but is dispersed, that is, light of different colors is bent by differing amounts and thus we have a separation of the various colors. If this takes place as the ray of light leaves the upper surface of a brilliant the observer upon whose eye the light falls will see either the red, or the yellow, or the blue, as the case may be, rather than the white light which entered the stone. If instead, the dispersion takes place as the light enters the brilliant the various colored rays thus produced will be totally reflected back to the observer (slightly weakened by spreading, as compared to the direct or unreflected spectra). Thus dispersion produces the "fire" in a brilliant.

Other materials than diamond behave similarly, but usually to a much smaller extent, for few gem materials have so high a refractive power or so great a dispersive power as diamond.

Having considered the theory of the brilliant we may now take up a study of the methods by which the exceedingly hard rough diamond is shaped and polished.

CLEAVING DIAMONDS. If the rough material is of poor shape, or if it has conspicuous defects in it which prevent its being made into a single stone, it is cleaved (_i. e._, split along its grain). Hard as it is, diamond splits readily in certain definite directions (parallel to any of the triangular faces of the octahedral crystal). The cleaver has to know the grain of rough diamonds from the external appearance, even when the crystals, as found, are complicated modifications of the simple crystal form. He can thus take advantage of the cleavage to speedily reduce the rough material in size and shape to suit the necessity of the case. The cleaving is accomplished by making a nick or groove in the surface of the rough material at the proper point (the stone being held by a tenacious wax, in the end of a holder, placed upright in a firm support). A thin steel knife blade is then inserted in the nick and a sharp light blow struck upon the back of the knife blade. The diamond then readily splits.

"CUTTING DIAMONDS." The next step is to give the rough material a shape closely similar to that of the finished brilliant but rough and without facets. This shaping or "cutting" as it is technically called, is done by placing the rough stone in the end of a holder by means of a tough cement and then rotating holder and stone in a lathe-like machine. Another rough diamond (sometimes a piece of bort, unfit for cutting, and sometimes a piece of material of good quality which it is necessary to reduce in size or alter in shape) is cemented into another holder and held against the surface of the rotating diamond. The holder is steadied against a firm support. It now becomes a case of "diamond cut diamond," each stone wearing away the other and being worn away itself.

The cutting process is fairly rapid and it leaves the stone (which is reversed to make the opposite side) round in form and with a rounding top and cone-shaped back. Stones of fancy shape, such as square, or cushion shape, have to be formed in part by hand rubbing or "bruting" as it is called.

The facets must now be polished onto the stone. Usually the workers who cut do not cleave or polish.

"POLISHING" DIAMONDS. The polisher fixes the cut stone firmly in a metallic holder called a dop, which is cleverly designed to hold the stone with much of one side of it exposed. The holder is then inverted so that the stone is beneath and a stout copper wire attached to the holder is then clamped firmly in a sort of movable vise. The latter is then placed on the bench in such a position that the diamond rests upon the surface of a rapidly revolving horizontal iron wheel or "lap" as it is called. The surface of the latter is "charged" with diamond dust, that is, diamond dust has been pushed into the metal surface which thus acts as a support to the dust. The latter wears away the diamond, producing a flat facet. The lap is kept moistened with oil and from time to time fresh oil and diamond dust are applied. A speed of about 2,000 rotations per minute is used.

FACETTING. The making of the facets is rather slow work, especially when, as is usually the case in making the "table" the work has to be done against one of the "hard points" of the crystal. Great care has to be taken to place the stone so that the grain lies in a correct position, for diamond cannot be polished against the grain, nor even exactly with it, but only obliquely across it. This requirement, as much as anything, has prevented the use of machines in polishing diamonds. The table is usually first polished on, then the four top slopes, dividing the top surface into quarters, then each of the four ridges thus left, is flattened, making eight facets and finally 32 facets, exclusive of the table, are made upon the top of the brilliant. The stone is then reversed and 24 facets, and the culet, polished on the back. As each facet nears its proper shape the stone is placed upon a particularly smooth part of the lap and a slight vibratory motion given to the holder by the hand. This smooths out any lines or grooves that may have formed because of inequalities of surface of the lap. When completely facetted the brilliant is finished and requires only to be cleaned, when it is ready for sale.

LESSON XXIII

HOW ROUGH PRECIOUS STONES ARE CUT AND WHAT CONSTITUTES GOOD "MAKE"--_Concluded_

SLITTING AND CLEAVING. The cutting and polishing of precious stones other than diamond is a trade entirely distinct from diamond cutting. The precious stone lapidary cuts every species of stone except diamond. The methods used by different lapidaries vary somewhat in their details, and there are many trade secrets which are more or less jealously guarded by their possessors, but in general the methods used to reduce the rough materials to the finished gems are as follows: First, the rough material, if of too large size, or if very imperfect, is _slitted_, or, if it possesses a pronounced cleavage, it may be _cleaved_, in order to reduce the size or to remove imperfect parts. _Slitting_ is accomplished by means of a circular disc of thin metal which is hammered so that it will be flat and rotate truly, and is then clamped between face plates, much as an emery wheel is held. The smooth edge of the circular disc is then charged with diamond dust and oil, the diamond dust being bedded into the edge of the metal disc by the pressure of some hard, fine-grained material, such as chalcedony, or rolled into the metal by the use of a rotating roller. Once charged, and kept freely supplied with oil, a slitting wheel will slice a considerable number of pieces of any precious stone less hard than diamond, and will do so with considerable rapidity. The wheel is, of course, rotated very rapidly for this purpose.

The cleaving of certain gem materials, such as true topaz (which splits perfectly across the prism, parallel to its base) is easily accomplished, and it is done in much the same manner as the cleaving of diamond. The feldspar gems, such as moonstone, amazonite, and labradorite, also cleave very smoothly in certain directions. Spodumene, of which Kunzite is a variety, cleaves almost too easily to be durable. Most gem minerals, however, lack such perfect cleavage and when it is desired to remove imperfect parts, or to reduce large pieces to smaller sizes, these materials are slitted as above described.

"RUBBING DOWN." The material being of nearly the dimensions of the finished piece, the next step is to "rub it down," as it is called, to approximately the shape and size desired. This rubbing down process was formerly done by means of a soft metal lap (sometimes of lead), charged with coarse emery powder and water. Carborundum, being harder and sharper than emery, has replaced it very largely. Some of the softer materials, such, for example, as turquoise, are rubbed down on a fast flying carborundum wheel of similar type to those used in machine shops for grinding steel tools. These wheels rotate in a vertical plane and are kept wet. The laps before mentioned run horizontally. The carborundum wheels have the grains of carborundum cemented together by means of some binding material and this gradually crumbles, exposing fresh, sharp cutting edges. Various sizes of grain, and various degrees of hardness of the binding material, as well as various speeds, are needed to suit the many different materials rubbed down by the lapidary. Some lapidaries rub down the harder and more valuable gems such as ruby upon diamond charged laps of brass or other metal.

CABOCHONS. The rubbing down process does not leave a facetted surface, but only a coarse roughly rounded or flattened surface. If the material is to be left in some one of the flat-backed, rounded top forms known as cabochon cut, the surfaces need only to be smoothed (by means of very fine abrasives such as fine emery applied by means of laps, or even by fine emery or carborundum cloth), and they are then ready for polishing.

FACETTED STONES. If, however, the stone is to be facetted in either the brilliant form, somewhat like the diamond, or step cut or otherwise facetted, it is cemented strongly onto a holder (much like the wooden part of a pen holder). The upper end of the holder is rested in one of a series of holes in what is called a "_ginpeg_" resting in the work-bench near a metal lap, and the stone is pressed upon the rapidly rotating surface of the lap, which is charged with diamond dust or carborundum, according to the hardness of the material to be facetted. A flat facet is thus ground upon the stone. By rotating the holder a series of facets, all in the same set, is produced. The holder is then changed to a new position on the ginpeg and another set of facets laid upon the stone. Thus as many as four or five tiers or sets of facets may be applied to one side, say the top of the stone. The latter is then removed from the holder and cemented to it again, this time with the bottom exposed, and several sets of facets applied.

The stone is now _cut_ but not _polished_. The facets are flat, but have a rough ground-glass like surface. The polishing is usually done by workers who do not cut stones, but who do nothing but polish them. In small shops, however, the same lapidary performs all the parts of the work.

POLISHING. The polishing of stones, whether cabochon or facetted, is accomplished by the use of very finely powdered abrasives such as corundum powder, tripoli, pumice, putty powder, etc. Each gem material requires special treatment to obtain the best results. It is here that most of the trade secrets apply.

The troubles of the lapidary in getting the keen polish that is so much admired on fine gems are many. In general, the polishing powder should not be quite as hard as the material to be polished, else it may grind rather than polish. The material should be used with water or oil to give it a creamy consistency. It should be backed by laps of different materials for different purposes. Thus, when backed by a fairly hard metal even tripoli, although much softer, will polish sapphire. On a lap of wood, tripoli would fail to polish hard materials, but would polish amethyst or other quartz gem. A change of speed of the lap, too, changes the effect of the polishing material. I have seen a lapidary, who was having no success at polishing an emerald, get very good results by using a stick as a brake and slowing down his lap.

The polishing material must be of very uniform size, preferably water floated or oil floated, to give good results. The lap must be kept flat and true and the stone must be properly held, or the flatness of the facets, upon which brilliancy depends in part, will be destroyed during the polishing.

The softer materials, such as opal, require treatment more like that accorded cut glass, and soft abrasive powders, such as pumice, suffice to polish them. Probably hardly two lapidaries would work exactly alike in their treatment of precious stones, and each guards his secrets, yet all use approximately similar general methods. Some have devised mechanical holders which permit the repeated cutting of stones to exactly the same angles, and that, too, with an accurate knowledge of the angles used. These angles can be definitely altered for different materials, according to their refractive indices. Other lapidaries produce very fine results by purely hand methods.

These details have been gone into to give an idea of the methods of the lapidary and of the many variations in method. In general, however, the _slitting_ or _cleaving_, the _rubbing down_ to shape, the _smoothing out_ of all scratches and the _facetting_ and _polishing_ are done somewhat similarly by all lapidaries.

Having now had a glimpse of the methods of the lapidaries, let us briefly consider what constitutes good "make" in stones other than diamond.

GOOD "MAKE" IN COLORED STONES. Brilliants, cut from materials having smaller refractive indices than diamond, (and this group includes nearly all stones other than diamonds) should have steeper back angles and higher tops than the best diamond brilliants have. A 35-degree top angle (the angle between the slope of the top and the plane of the girdle is called the top angle) and a 41-degree back angle being about ideal for diamond, other gem materials should have more nearly a 39-degree top angle and a 44-degree back angle to give the greatest possible brilliancy. However, in the case of colored gems such as ruby, sapphire, etc., where the value depends even more largely upon the color than upon the brilliancy, it is frequently necessary to cut the brilliant thicker or thinner than these proportions in order to deepen or to thin the color.

In general, the thicker a stone of a given spread the deeper the color will be. The color may also be deepened by giving to the stone a rounded contour, both above and below the girdle, and facetting it in steps instead of in the brilliant form. Increasing the number of steps also serves to slightly deepen the color, as a larger number of reflections is thus obtained within the material, the light thus has to travel a greater distance through the colored mass, and more of the light, of color other than that of the stone, is absorbed.

IMPROVING COLOR BY PROPER CUTTING. In addition to the color improvement that can be brought about by changing the shape of the cut stone there are a number of gem materials whose color varies very greatly in different directions, and this fact calls for skillful use in order to obtain the best possible results. Thus most tourmalines of deep color must be cut with the top or table, of the finished stone, on the _side_ of the prismatic crystal rather than at right angles to the axis of the prism. If cut the latter way they would be much too dense in color. On the other hand, most blue sapphires should be cut _across_ the prism axis rather than the way that tourmalines should be cut. To cut a sapphire with its table on the side of the prism would be likely to cause it to have a greenish cast because of the admixture of the unpleasing "ordinary ray" of yellowish tint with the blue of the stone as seen up and down the prism. Some Australian sapphires are of a pronounced green when viewed across the axis of the crystal.

Rubies if cut, as was recommended for sapphires, give a very pure and very deep red color, but lack somewhat in the display of dichroism given by rubies that are cut with the table on the side of the crystal and parallel to its axis. Lapidaries need to know and to make use of such optical relations as these and jewelers might well inform themselves in such matters, especially if they have, or hope to acquire, trade in very fine colored stones.

EFFECT OF SHAPE ON BRILLIANCY. In actual practice it is common to find colored stones poorly cut for brilliancy, especially central brilliancy, and that, too, without the excuse of sacrifice of brilliancy in order to improve color. The fault is usually due to too great a desire to save size and weight. Frequently a stone would have greater value if properly cut, even at the expense of some size and weight. When stones are cut too shallow, as is frequently the case, they are sure to leak light in the center and they are thus weak and less brilliant there than they would be if made smaller in diameter and with steeper back slopes approximating 44 degrees.

Round stones, if their angles are correct, are more brilliant than stones of other contour such as square or cushion shape, or navette or heart shape. It can readily be seen that such odd-shaped stones can hardly have the same top and back angles at every part of their circumference. If the angle from a corner of a square stone is correct then the angle from the middle of one side is obviously a little different. Small differences of angle make considerable differences in the brilliancy of cut stones. The prevailing tendency to cut nearly all diamonds round depends largely upon the above facts. In the case of colored stones, however, the added attractiveness which comes with odd or different contour more than makes up for the slight loss of brilliancy that may attend upon the shape selected. Such shapes as lend themselves to special designs in mountings also justify any little loss in brilliancy that accompanies the change in shape, provided the proportions retained give a considerable amount of total reflection within the stone and thus light up most of the stone as seen from the front.

The test of the "make" of a color stone is its appearance. If it lights up well over most of its surface and if the color is right, one should not criticize the "make" as one would be justified in doing in the case of a diamond. If, however, the effect is less attractive it would many times be advisable to measure the angles of the stone, or its thickness and spread as compared with similar measurements on a stone of fine appearance. Frequently one will thus find the reason for the failure of the stone to perform as it might, and recutting should be resorted to in such cases in order to get a smaller but more beautiful and hence more valuable stone.

LESSON XXIV

FORMS GIVEN TO PRECIOUS STONES

While precious stones are cut to many different forms, there are, nevertheless, but a few general types of cutting. These may be classified as follows: First, the "_cabochon_" (Fig. 11) type of cutting; second, the old "_rose_" (Fig. 12) type of cutting; third, the _brilliant_ (Fig. 13); fourth, the _step cutting_ (Fig. 14).

CABOCHONS. Of these the first, or _cabochon_ cutting, is probably the most ancient. The term comes from a French word signifying a bald pate (caboche, from Latin cabo, a head). The usual round cabochon cut closely resembles the top of a head in shape. Cabochon cut stones usually have a flat base, but sometimes a slightly convex base is used, especially in opals and in moonstones, and some stones of very dense color are cut with a concave base to thin them and thus to reduce their color. The contour of the base may be round, or oval, or square, or cushion shape, or heart shape or of any regular form. The top is always smooth and rounding and unfacetted. The relation of the height or thickness to the length or width may be varied to suit the size and shape of the rough piece or to suit one's ideas of symmetry, provided the material be an opaque one, such as turquoise or lapis lazuli. If, however, the material is transparent the best results in the way of the return of light to the front, and hence in the display of the color of the material, are had if the thickness is about one half the spread.

This relation depends upon the refractive index of the material, but as most color stones are of somewhat similar refractive indices, the above proportions are sufficiently accurate for all. The object in view is the securing of total reflection of as much light as possible from the flat polished back of the stone. Cabochon stones are sometimes set over foil or on polished gold to increase the reflection of light.

The path of a ray of light through a cabochon cut stone is closely similar to that through a rose cut diamond [see cut (c) of Fig. 12 for the latter.] Like the rose cut, the cabochon cut does not give much brilliancy as compared to the brilliant cut. Cabochon cut stones, however, have a quiet beauty of color which commends them to people of quiet taste, and even fine rubies, sapphires, and emeralds are increasingly cut cabochon to satisfy the growing demand for fine taste in jewels. The East Indian has all along preferred the cabochon cut for color stones, but possibly his motives have not been unmixed, as the cabochon cut saves a greater proportion of the weight of the rough stone than the more modern types of cutting.

Garnets, more than other stones, have been used in the cabochon cut, and when in that form are usually known as _carbuncles_ (from carbunculus, a glowing coal). Any other fiery red stone might equally well be styled a carbuncle, especially if cabochon cut.

Scientific rubies look very well in the cabochon cut.

Fig. 11 shows in (a) and (b) the front and top of the usual round cabochon. Cut (c) of the same figure gives the front elevation of a cabochon which will light up better than the usual round-topped design. In the round-topped type the central part of the top is so nearly parallel to the back that light can pass right through as through a window pane. If the sloping sides are brought up to a blunt point, as in cut (c) there is very much less loss of light and greater beauty results. The East Indian cabochons are frequently cut in a fashion resembling that suggested.

ROSE CUT STONES. It was natural that the earliest cut stones should have the simple rounded lines of the cabochon cutting, for the first thing that would occur to the primitive worker who aspired to improve upon nature's product, would be the rubbing down of sharp edges and the polishing of the whole surface of the stone. Perhaps the next improvement was the polishing of flat facets upon the rounded top of a cabochon stone. This process gives us the ancient type of cutting known as the _rose_ cut. The drawings (a) and (b) of Fig. 12 show the front elevation and the top and (c) shows the path of a ray of light through a "rose." It will be noted that the general shape resembles that of a round cabochon, but twenty-four triangular facets have been formed upon the top. The well-proportioned rose has a thickness about one half as great as its diameter. Diamonds were formerly cut chiefly in the rose form, especially in the days of the East Indian mines, and even in the early part of the nineteenth century many people preferred finely made roses to the thick, clumsy brilliants of that day. To-day only very small pieces of diamond are cut to "roses." As the material so used frequently results from the cleaving of larger diamonds, the public has come to know these tiny roses as "chips."

The best roses have twenty-four regular facets but small ones frequently receive only twelve, and those are seldom regular in shape and in arrangement. Such roses serve well enough for encrusting watch cases and for similar work, as the flat base of the stone can be set in thin metal without difficulty. About the only gem other than diamond that is now cut to the rose form is garnet. Large numbers of small Bohemian garnets are cut to crude rose form for use in cluster work.

THE BRILLIANT CUT, as its name implies, gives the most complete return of light of any of the forms of cutting. The theory of the brilliant has already been discussed (Lesson XXII. in connection with the cutting of diamond). The shape of the brilliant is too well known to require much description. Most brilliants to-day are cut practically round and the form is that of two truncated cones placed base to base. The upper cone is truncated more than the lower, thus forming the large, flat top facet known as the _table_ of the stone [A, Fig. 13, cut (a)]. The truncating of the lower cone forms the tiny facet known as the culet, which lies opposite to the table and is parallel to the latter [see B, Fig. 13, cut (a)]. The edge of meeting of the two cones is the _girdle_ of the brilliant [CD in cut (a), Fig. 13]. The sloping surface of the upper cone is facetted with thirty-two facets in the full cut brilliant, while the lower cone receives twenty-four.

Small stones sometimes receive fewer facets, to lessen the cost and difficulty of cutting, but by paying sufficient for them full cut brilliants as small as one hundred to the carat may be had. Cut (b) of Fig. 13 shows the proper arrangement of the top facets and cut (c) that of the bottom facets.

When cutting colored stones in the brilliant cut, especially if the material is very costly and its color in need of being darkened or lightened, the lapidary frequently takes liberties with the regular arrangement and proportions depicted in the cuts.

STEP CUTTING. The only remaining type of cutting that is in very general use is the _step cut_ (sometimes known as trap cut). Fig. 14, (a), (b), and (c), shows the front elevation, the top and the back of a square antique step cut stone. The contour may be round or completely square or oblong or of some other shape, just as a brilliant may have any of these contours. The distinctive feature of the step cutting is the several series of parallel-edged quadrangular facets above and below the girdle and the generally rounding character of its cross section. This plump, rounding character permits the saving of weight of the rough material, and by massing the color gives usually a greater depth of color than a brilliant of the same spread would have if cut from similar material. While probably never quite as snappy and brilliant as the regular brilliant cut, a well-proportioned step cut stone can be very brilliant. Many fine diamonds have recently been cut in steps for use in exclusive jewelry.

THE MIXED CUT. The ruby and the emerald are never better in color than when in the full step cut, although rubies are frequently cut in what is known as the _mixed_ cut, consisting of a brilliant cut top and a step cut back. Sapphires and many other colored stones are commonly cut in the mixed cut. Recently it has become common to polish the tops of colored stones with a smooth unfacetted, slightly convex surface, the back being facetted in either the brilliant or the step arrangement. Such stones are said to have a "_buffed top_." They are less expensive to cut than fully facetted stones and do not have the snappy brilliancy of the latter. They do, however, show off the intrinsic color of the material very well.

LESSON XXV

IMITATIONS OF PRECIOUS STONES

"PASTE" GEMS. Large volumes have been written on paste jewels, especially on antique pastes. Contrary to a prevailing belief, the paste gem is not a recent invention. People frequently say when told that their gems are false, "But it is a very old piece, it must be genuine." The great age of a jewel should rather lead to suspicion that it was not genuine than give confidence that a true gem was assured. The Egyptians and Romans were skillful makers of glass of the sort used in imitating gems and some of the old pastes were very hard or else have become so with age.

Glass of one variety or another makes the most convincing sort of imitation precious stones. The term "paste" as applied to glass imitations is said to come from the Italian _pasta_ meaning dough, and it suggests the softness of the material. Most pastes are mainly lead glass. As we saw in Lesson XVIII., on the chemical composition of the gems, many of them are silicates of metals. Now glasses are also silicates of various metals, but unlike gem minerals the glasses are not crystalline but rather amorphous, that is, without definite geometric form or definite internal arrangement.

The optical properties of the various glasses vary chiefly with their densities, and the denser the material the higher the refractive index and the greater the dispersion. Thus to get the best results in imitation stones they should be made of very heavy glass. The dense flint glass (chiefly a silicate of potassium and lead) which is used for cut glass ware illustrates admirably the optical properties of the heavy glasses. By using even more lead a still denser glass may be had, with even a greater brilliancy.

Unfortunately the addition of lead or other heavy metals (such as thallium) makes the product very soft and also very subject to attack by gases such as are always present in the atmosphere of cities. This softness causes the stones to scratch readily so that when worn they soon lose their polish and with the loss of polish they lose their beauty. The attack of the gases before mentioned darkens the surfaces of the imitation and further dulls it. When fresh and new a well cut piece of colorless paste has a snap and fire that approaches that of diamond. The surface luster is not adamantine, however, and the edges of the facets cannot be polished so sharply as those on a diamond. Moreover the refractive index, while high, is never so high as in a diamond and hence the brilliant cannot be so shaped as to secure the amount of total reflection given by a well-made diamond. Hence, the paste brilliant, while quite satisfying as seen from squarely in front, is weak and dark in the center as seen when tilted to one side. By these differences the trained eye can detect paste imitations of diamond at a glance without recourse to tests of specific gravity, hardness, etc.

Pastes, being amorphous, are singly refracting, as is diamond. This fact helps the appearance of the paste brilliant, for light does not divide within it to become weakened in power. This singleness of refraction, however, betrays the paste imitation when it is colored to resemble ruby, sapphire or emerald, all of which are doubly refracting.

The color is imparted to pastes by the addition, during their manufacture, of various metallic oxides in small proportions. Thus cobalt gives a blue color, copper or chromium green, copper or gold give red (under proper treatment) and manganese gives purple. By experiment the makers of pastes have become very skillful in imitating the color of almost any precious stone. Fine paste emeralds may look better than inferior genuine emeralds.

As pastes are singly refracting and hence lack dichroism, the pleasing variety of color of the true ruby cannot be had in a paste imitation, but the public is not critical enough to notice this lack. The expert would, however, note it and could detect the imitation by that difference as well as by the lack of double refraction. The use of direct sunlight and a white card as already explained in the lesson on double refraction (Lesson III.) will serve to expose the singleness of refraction of paste imitations. Spinels and garnets are about the only true gems (except diamond) that are single refracting. Any other color stone should show double refraction when tested by the sunlight-card method. The file test will also expose any paste imitation as all the very brilliant pastes are fairly soft.

DOUBLETS. To give better wearing quality to paste imitations the _doublet_ was devised. This name is used because the product is in two parts, a lower or back portion of paste and an upper or top portion of some cheap but hard genuine stone. Garnet is probably used for this purpose to a greater extent than any other material, although quartz or colorless topaz will do very well.

The usual arrangement of the parts can be seen in Fig. 15, the garnet covering only a part of the upper surface, namely the table part and a small portion of the sloping surface of the top. In high class doublets the hard mineral covers the paste to the girdle. (See Fig. 16.) The color of the garnet does not interfere seriously with that of the paste.

If a "diamond" doublet is desired the slice of garnet is made nearly as thin as paper and it covers only the table of the brilliant. It is thus practically colorless. A thin slice of red garnet over a green background is not noticeable, as all the red is absorbed in passing through the green material beneath. With a blue base, the red upper layer may give a very slight purple effect. With yellow a slight orange tint results and of course with a red back no perceptible difference would result.

The two materials are cemented together, by means of a transparent waterproof cement. The _triplet_ has already been described in Lesson XII. It is even better than the doublet and more difficult to detect. Both the file test and the sunlight-card test serve to detect doublets, as well as paste imitations, except that in the file test with the fully protected doublet the _back_ of the stone must be tested with the file, as the girdle and top are of hard material.