Chapter III: Introduction (2)

In Texas, pearls have been found in fresh-water clams in most of the major rivers and streams, notably in the Brazos, Concho, Colorado, Guadalupe, Llano, Nueces, Sabine, Rio Grande, and Trinity Rivers. Several Texas lakes have also yielded pearls, notably Caddo Lake and other lakes in north-central and northeast Texas.

Small pearls are frequently found along the Texas Gulf Coast in edible oysters and other common shellfish. Fossil pearls have also been found but because of their darkened appearance are of value only as curiosities.

The pearls thus far found in Texas have been of relatively poor quality and show little or no iridescence. These pearls have little value except as curiosities, although one writer has stated that the discovery of pearls in the Nueces River led to the original Spanish settlement of the State (Baker, 1935, p. 569).

Quartz

_Composition_: SiO₂. _Crystal system_: hexagonal. _Hardness_: 7.
_Specific gravity_: 2.65 to 2.66 in crystals. _Luster_: vitreous, also
waxy, greasy, and dull. _Color_: most often colorless, brown, yellow,
violet; sometimes green, red, blue, and black; cryptocrystalline
varieties often variously colored by impurities. _Streak_: white.
_Cleavage_: indistinct. _Fracture_: conchoidal to splintery.
_Tenacity_: brittle to tough. _Diaphaneity_: transparent to opaque.
_Refractive index_: 1.544 to 1.553.

The quartz family gemstones can be divided into two groups for purposes of description. The first group is the crystalline varieties, or those quartz varieties that commonly occur in distinct crystals. The second group is the cryptocrystalline varieties, or those quartz varieties that occur as irregular masses that are composed of many microscopic crystals. The crystalline varieties are usually much more transparent and are most often seen as faceted stones. The cryptocrystalline varieties vary from subtransparent to opaque and are almost always cut as cabochons.

CRYSTALLINE VARIETIES

_Amethyst_ (violet to purple-colored quartz).—A northeastern Gillespie County locality known as Amethyst Hill has produced quite a number of fine light to medium violet amethyst crystals which occur in quartz veins and geodes associated with serpentine and talc. Many crystals have been found loose in the soil.

The amethyst tends to be very irregularly colored in zones parallel to the crystal faces. In many, the base of the crystal is colorless or white and only the termination is violet. Crystals up to 3 inches long have been found at this locality, but the average size is much less.

The surface at this locality is almost entirely depleted of amethyst, with only an occasional small crystal or fragment to be seen. However, small excavations are still sometimes productive.

Good groups of pale amethyst crystals have been found in quartz veins near the old town site of Oxford, Llano County. The occurrence seems to be much the same as the Amethyst Hill locality. Little exploration for gemstones has been done in this area, and future discoveries seem likely.

Chalcedony geodes lined with amethyst crystals have been found in Brewster, Presidio, Culberson, and Hudspeth counties, but the occurrences are scattered. The crystals are seldom large enough to yield gems of more than 3 carats and are mostly very light colored.

A few pieces of gem-quality amethyst have been found in Burnet County.

_Citrine_ (yellow quartz).—Very little gem-quality citrine has been reported in Texas. Some small citrine crystals have been found at Amethyst Hill in northeastern Gillespie County, but few are of sufficient size or color to yield good gems.

The writer has seen one citrine crystal that was found in the gravels of a small stream in eastern Llano County near Buchanan Dam. The crystal weighs about 1 ounce and is perfectly clear, light golden yellow, and flawless. However, a further search of the stream gravels failed to produce any other citrines.

_Rock crystal_ (colorless quartz).—Numerous localities in Texas produce this colorless variety of quartz, which is the most common variety of facet quality quartz and consequently is of little value.

Rock crystal occurs at many localities in Burnet, Llano, and Mason counties. The crystals mostly occur in pegmatite dikes or in stream gravels where they have been weathered out of their parent rock. Some fine colorless quartz crystals have been found near Voca, Mason County, in weathered pegmatite dikes and also loose in the sands of nearby streams. Crystals from this locality are often stained with reddish iron oxide on their outer surfaces. Some of the rock crystal found near Katemcy, Mason County, shows asterism when cut with the proper orientation. Fine clear colorless crystals up to 8 inches long have been found in the pegmatite dikes near Lake Buchanan in both Llano and Burnet counties. Several localities near Enchanted Rock in Llano County have also produced some good colorless crystals.

Feldspar quarries in large pegmatites in northeastern Gillespie County have yielded attractive quartz crystals, some of which contain smoky phantom crystals and tourmaline inclusions.

Some pieces of rock crystal enclosing green, needle-like actinolite crystals have been found near the Llano-Gillespie-Blanco County corner. This material is not suitable for faceted gems but does lend itself to interesting and attractive cabochons.

Colorless quartz crystals commonly are found lining small chalcedony geodes in Brewster, Presidio, Culberson, Hudspeth, Reeves, and Jeff Davis counties. These crystals are most commonly less than 1 inch long but are mostly very clear.

Rock crystal has been found in crevices of petrified wood in many east and southeast Texas counties, although the crystals are mostly quite small.

Many lesser occurrences of rock crystal, too numerous to mention, are located within the State.

_Rose quartz_ (pink quartz).—Rose quartz occurs at various localities in Burnet, Llano, Mason, and Gillespie counties, but the amount of material is mostly small and the greater part unsuitable for gem purposes. Some good pink rose quartz occurs near Town Mountain, Llano County, but this material does not have flawless areas large enough to yield faceted stones of more than a few carats. Rose quartz is always slightly milky, or cloudy, and does not cut into brilliant faceted stones. The Town Mountain rose quartz has been cut into attractive cabochons.

_Smoky quartz_ (brown, yellow-brown, and golden-brown quartz).—Several Texas localities have produced fine smoky quartz. Baringer Hill, a noted rare-earth minerals pegmatite locality in Llano County, contained some smoky quartz crystals that were estimated to weigh over 1,000 pounds, and the locality produced many smaller crystals that were of gem quality. Baringer Hill was flooded by the completion of Buchanan Dam in 1938 and is presently under the waters of Lake Buchanan. A few fine golden-brown gem-quality crystals have been found along the lake shore and in small pegmatites nearby (Pl. III, B.).

Feldspar quarries in northeastern Gillespie County have produced smoky quartz crystals that exceed 1 foot in length, but these crystals are mostly flawed, possibly as a result of blasting, and mostly contain only small clear areas.

Good color smoky quartz crystals are found with topaz in the pegmatites and stream beds in Mason County, near Streeter, Grit, and Katemcy. These crystals tend to be lighter colored than those near Lake Buchanan, but they commonly contain large flawless areas.

CRYPTOCRYSTALLINE VARIETIES

_Chalcedony._—When free from impurities of various oxides and other compounds, chalcedony has little to render it pleasing as a gemstone. It is mostly gray, white, brown, or bluish and commonly has a waxy luster. Some of the chalcedony found along the Rio Grande Valley and in west Texas will take dyes, and local lapidaries have had some success in dyeing this material various shades of blue, green, yellow, and red. When the chalcedony is naturally colored and variegated, usually in bands, mossy figures, or dendritic forms, it is called agate.

_Agate_ (variegated chalcedony).—The wide variety of markings and colors available together with the ease of cutting make agate a favorite of many lapidaries. Fine agate has been found at numerous localities in west and south Texas. Fine plume agate, famous throughout the United States, is found south of Alpine. Plume agate is characterized by dendritic or tree-like inclusions and is mostly cut into very handsome cabochons. The agate from south of Alpine commonly contains black, red, yellow, or brown plumes within the same piece. The variety of colors and lack of porosity of this agate make it highly desired among lapidaries. The agate occurs loose on the surface of the ground and in the soil in small nodules that have a very rough, brownish surface. These nodules are mostly less than 3 inches in diameter, although specimens of gem quality have been found that exceed 200 pounds.

Some very fine agate has been found in the vicinity of Needle Peak, Presidio County. This material is mostly green moss agate in clear chalcedony and commonly contains small yellow “sun-burst” figures. The contrasting yellow and green design makes very beautiful cabochons.

Fine agate has been found south of Marfa, Presidio County. This agate is mostly clear chalcedony with black, yellow, or variously colored plumes, moss, or “bouquet-like” figures.

Numerous other localities in Presidio and Brewster counties have produced good agate.

Various amounts of agate, jasper, and chalcedony occur in the gravels of the Rio Grande in varying quantities from Big Bend National Park downstream to Brownsville. This agate is found both in the present river gravels and in the older river gravels that now are located on nearby hills and slopes up to several miles north or south of the present Rio Grande. The greatest concentration of agate and related gem materials seems to be in the area between Laredo and Rio Grande City. Vast quantities of excellent gem material have been removed from this area for many years (Pl. IV). The agate occurs as rounded, stream-worn cobbles and commonly has a thin white coating that makes it difficult to distinguish from the abundant chert and other rocks. The agate occurs in cobbles that are mostly 3 to 6 inches in diameter, but specimens of gem quality that exceed twice this size are known. The agate varies greatly in design and color. Plume, moss, banded, and sagenitic agate occur in these gravels in a wide variety of colors. The jasper in the Rio Grande gravels is yellow, red, green, or various shades of these and is commonly suspended as angular fragments in clear chalcedony.

Good agate has also been found near Balmorhea in Reeves and Jeff Davis counties and in smaller amounts at numerous other west and south Texas localities.

_Agatized wood_ (_see_ Fossil wood, pp. 20-21).

_Carnelian_ (translucent reddish chalcedony).—This variety of chalcedony in small quantities has been reported from near Van Horn, Hudspeth County. Small pieces of carnelian have been found in the gravels of the Rio Grande, but finds have been few and scattered.

_Jasper_ (impure opaque or subtranslucent quartz).—Good green, yellow, red, and brown jasper has been found in the gravels of the Rio Grande at all of the localities that produce agate. The colors are quite vivid, and the material takes a fine polish. Some pieces of orbicular jasper (jasper with circular or eye-like markings) have been found in this material. These gravels commonly contain jasper as fragments that are suspended in clear chalcedony; this is called brecciated jasper and yields very handsome cabochons.

Many of the west Texas agate localities also produce jasper in quantity. Good jasper has been reported from north of Brackettville, Kinney County. Jasper is a minor constituent of the stream gravels in many parts of the State.

Sanidine

_Composition_: KAlSi₃O₈; commonly contains some sodium. _Crystal
system_: monoclinic. _Hardness_: 6. _Specific gravity_: 2.57 to 2.58.
_Luster_: vitreous to pearly. _Color_: colorless, white, pale yellow,
and gray. _Streak_: uncolored. _Cleavage_: three directions.
_Fracture_: conchoidal to uneven. _Tenacity_: brittle. _Diaphaneity_:
transparent to subtranslucent. _Refractive index_: 1.52 to 1.53.

Some feldspars, including sanidine, show a nice blue sheen in reflected light parallel to certain crystallographic directions. Stones having this property are called moonstone. A clear yellowish sanidine showing an attractive blue sheen has been found in Brewster, Jeff Davis, and Presidio counties. The individual pieces are small, the average size being about one-eighth inch. The sanidine is found loose in the soil at some localities where it has weathered out of rhyolite, and specimens of the sanidine in the parent rock are not difficult to obtain. Very small cabochons can be cut from this material, but few lapidaries have done so because inexpensive larger pieces of moonstone can be obtained easily from foreign sources. However, the west Texas sanidine does show a blue sheen when cut and polished.

Spinel

_Composition_: MgAl₂O₄ (magnesium may be replaced in part by ferrous
iron or manganese and the aluminum by ferric iron and chromium).
_Crystal system_: isometric. _Hardness_: 8. _Specific gravity_: 3.5 to
4.1. _Luster_: vitreous to sub-metallic. _Color_: black, pink, red,
blue, green, yellow, brown, and violet. _Streak_: white. _Cleavage_:
one direction, imperfect. _Fracture_: conchoidal. _Tenacity_: brittle.
_Diaphaneity_: transparent to opaque. _Refractive index_: variable,
approximately 1.72 to 2.00.

In many areas of the world, fine quality, beautifully colored, transparent spinels are found and used as gems. The only gem-quality spinel reported thus far in Texas is black and opaque. Near Eagle Flat in Hudspeth County, black spinel crystals have been found associated with augite and natural glass; these minerals are weathering out of an intrusive igneous rock. The spinel crystals have an octahedral form which is common for this mineral (fig. 17). Most of the spinels are free of flaws, but because of their black color they have little value as gems. The crystals are found loose in the sand of streams near the outcrops of the igneous rock or embedded in the rock. They seldom exceed half an inch in diameter. These stones are primarily sought by collectors.

Tektite (Bediasite)

_Composition_: A natural glass, approximately 75% SiO₂, 15% Al₂O₃, 4%
FeO, also MgO, Na₂O, K₂O, and traces of other elements. _Crystal
structure_: amorphous. _Hardness_: 5 to 6. _Specific gravity_: 2.33 to
2.44. _Luster_: vitreous, often dull on weathered surfaces. _Color_:
dark brown, greenish brown, appears black in thick sections. _Streak_:
uncolored. _Cleavage_: none. _Fracture_: conchoidal. _Tenacity_:
brittle. _Diaphaneity_: transparent to subtransparent. _Refractive
index_: 1.488 to 1.512.

The average bediasite size is about 1 inch in diameter, although specimens approximately 3 inches in diameter are known. The uncut tektites are very interesting, showing a variety of shapes and surface features (Pl. V, A) and many exhibit contorted flow structure. The surface of many tektites is grooved or furrowed, while on others it is smooth or frosted. The Texas tektites are known as “bediasites,” after place names in Grimes County traceable to the Bedias Indians who formerly lived there.

Dark brown and greenish-brown tektites have been found in Texas in gravels at scattered localities in Walker, Grimes, Brazos, Burleson, Lee, Fayette, Gonzales, Lavaca, and DeWitt counties. Outside of Texas the only other authenticated tektite localities in the United States at the present time are in Dodge and Irwin counties, Georgia. A fragment of a similar tektite has recently been reported from near Martha’s Vineyard, Massachusetts. The tektites reported from Oklahoma are now known to be pebbles of obsidian.

Although tektites have little value or beauty as gemstones, they have been cut by lapidaries as both faceted and cabochon stones. Tektites take a high polish but are mostly so dark in color that they appear black.

The origin of tektites is of great scientific interest and is currently the subject of much debate. Some scientists believe that tektites are of meteoritic origin, while others believe that tektites were formed by various terrestrial processes. Since no one has actually observed a tektite to fall or form, and many of the theories of origin are difficult to prove without direct observation, the origin of tektites is likely to remain in controversy for some time.

Topaz

_Composition_: Al₂(F, OH)₂SiO₄. _Crystal system_: orthorhombic.
_Hardness_: 8. _Specific gravity_: 3.4 to 3.6. _Luster_: vitreous.
_Color_: pale blue, sky blue, greenish, white, wine yellow, straw
yellow, grayish, pink, reddish, and orange. _Streak_: uncolored.
_Cleavage_: one direction, basal, highly perfect. _Fracture_:
conchoidal to uneven. _Tenacity_: brittle. _Diaphaneity_: transparent
to subtranslucent. _Refractive index_: about 1.60 to 1.63.
_Dispersion_: moderate.

Various yellow and smoky colored quartz gems are offered for sale as “Spanish Topaz,” “Smoky Topaz,” “Madeira Topaz,” and “Topaz Quartz.” These names are entirely misleading and should be dropped from usage.

Fine gem-quality white, pale-blue, and sky-blue topaz has been found near Streeter, Grit, and Katemcy, Mason County. This Texas gem material compares favorably in color, size, and clarity with topaz found anywhere in the United States. Fine crystals of topaz (Pl. V, B, and fig. 18) occasionally are found in pegmatite dikes associated with quartz, black tourmaline, cassiterite, and pink microcline. Many of the gem-bearing pegmatites have been eroded away, leaving the topaz concentrated in the stream beds. The stones mostly occur as frosted, stream-worn pebbles (Pl. VI, A) in the numerous small creeks in the area. The topaz is heavier than the quartz and microcline that compose the stream gravel and is commonly found immediately on top of the granite bed-rock in the bottom of the stream bed. The stones tend to lodge behind boulders or small dikes cutting across the stream.

The white or colorless stones are by far the most common, outnumbering the bluish stones about ten to one. The color of the blue stones tends to be irregularly distributed in zones parallel to the crystal faces. Topaz that is colored in this manner should be cut with the best blue color near the bottom or culet of the gem (fig. 19). If done correctly, this will give the entire gemstone the desirable blue color.

COLORLESS
BLUE

The colorless stones can be turned pale yellow, yellowish brown, or straw yellow by exposure to X-ray radiation, and some of the bluish stones will fluoresce faintly yellowish under ultra-violet light.

The largest gem-quality topaz crystal yet found in North America has come from Mason County. It is a pale-blue crystal weighing 1,296 grams, now in the collection of the U.S. National Museum. Several other large pieces, some weighing over a pound, have been found. One large crystal, exact weight unknown, was found near Katemcy. Several gem cutters have estimated that this stone could easily yield a single, flawless pale-blue gem of about 500 carats. Many large gems have been cut from topaz found in this area, including at least one stone of over 300 carats.

One obstacle in the cutting of topaz is its perfect basal cleavage. The gemstone should be oriented so that no facet of the stone will be parallel to or within less than about 5 degrees of the cleavage direction, or the facet may be very difficult or impossible to polish.

It is difficult to estimate the productivity of this area since its discovery in the early 1900’s. Few systematic attempts have been made to exploit the deposits, and a great amount of the topaz thus far recovered has been found by private collectors. The Mason County topaz deposits are still very productive, and additional exploration may uncover even more gem-producing areas.

Topaz has also been found in stream gravels or pegmatites in Burnet, Llano, Gillespie, and El Paso counties but very rarely in gem quality.

Tourmaline

_Composition_: H₉Al₃(B·OH)₂Si₄O₁₉; hydrogen often replaced by iron,
magnesium, calcium, or fluorine. _Crystal system_: hexagonal.
_Hardness_: 7 to 7.5. _Specific gravity_: 2.98 to 3.20. _Luster_:
vitreous to resinous. _Color_: black, brownish black, brown, blue,
green, red, pink, yellow, and gray. _Streak_: uncolored. _Cleavage_:
two directions, very imperfect. _Fracture_: subconchoidal to uneven.
_Tenacity_: brittle. _Diaphaneity_: transparent to opaque. _Refractive
index_: about 1.62 to 1.64.

Black tourmaline is schorl; brown tourmaline, dravite.

Good crystals of black and dark brown tourmaline occur at Town Mountain near Llano, Llano County. The tourmaline crystals average about 1 inch in length, do not commonly exceed 2 inches, and are associated with white vein quartz. The quartz completely encloses the tourmaline, but the crystals can be broken free or the quartz can be trimmed away with the use of a diamond saw. The latter procedure is recommended whenever possible, for it is very easy to shatter the tourmaline crystals while trying to remove them from the quartz by other means. Many of the crystals are completely unsuitable for cutting, being too brittle or too badly cracked and flawed. However, some small crystals have been found that are of sufficient quality and size to yield flawless stones of a few carats. Few of these stones have been cut since the tourmaline is so dark that it appears opaque, and few persons find a gem of this nature attractive.

Good black and dark brown crystals of tourmaline associated with andalusite and graphite occur in the Packsaddle schist (Precambrian) near Sunrise Beach, Llano County (Pl. VI, B, and fig. 20). Although generally smaller in diameter than the crystals found at Town Mountain, they commonly exceed 3 inches in length, although the average size is a little over 1 inch. Many of these crystals are suitable for cutting into opaque or nearly opaque stones of about 5 or 6 carats.

Black tourmaline has also been found in Hudspeth and Culberson counties but not of sufficient quality to be used as a gemstone.

Turquoise

_Composition_: hydrous phosphate of aluminum and copper. _Crystal
system_: triclinic. _Hardness_: 5 to 6. _Specific gravity_: variable,
2.6 to about 2.8. _Luster_: dull, sometimes waxy. _Color_: sky blue to
greenish blue. _Streak_: white to greenish. _Cleavage_: none in
massive material, two directions in crystals. _Fracture_: conchoidal
to subconchoidal. _Tenacity_: brittle. _Diaphaneity_: subtranslucent
to opaque. _Refractive index_: 1.61 to 1.65.

Turquoise of good sky-blue to greenish-blue color has been found a few miles southwest of Van Horn, Culberson County. Several shallow pits were dug at this locality about 1910; however, the amount of turquoise produced was small. The main occurrence of the turquoise was in seams about 1 millimeter thick along joints in the fine-grained rocks of this area. Persons who have visited Culberson County more recently report that even minute traces of the turquoise are now difficult to find at the old prospect pits. However, further prospecting in the area might yield some additional localities.

Small amounts of turquoise have been reported near El Paso, El Paso County, and also in volcanic rocks near the Jeff Davis-Brewster County line, north of Alpine.

A small amount of turquoise has been mined from several localities a few miles northwest of Sierra Blanca in the Sierra Blanca Mountains of Hudspeth County.

GLOSSARY

Amorphous—without definite molecular structure; not crystalline.
Baroque stone—an irregularly shaped, polished stone; usually applied
to tumbled stones.
Baroque pearl—an irregularly shaped pearl.
Brilliancy—reflecting much light; having brightness.
Brilliant cut—a mode of arrangement of facets commonly used on round
or oval stones. The standard American brilliant cut has 57 or
58 facets. Most diamonds of 5 or less carats are cut in this
manner.
Cabochon—a stone cut with a flat or convex upper surface; sometimes
faceted in part. Opal, star sapphire, and agate are stones
that are frequently cut in this style (fig. 2).
Cambrian—a division of geologic time, estimated to be the time from
550 to 440 million years ago; the oldest time division of the
Paleozoic era.
Carat—a unit of weight equal to ⅕ of a gram or 0.2 gram. One ounce
avoirdupois is equal to 141.75 carats.
Cleavage—the tendency of certain minerals to split in particular
directions yielding relatively smooth plane surfaces.
Conchiolin—an organic albuminoid substance found in pearls.
Conchoidal—a type of fracture having curved concavities or the
approximate shape of one-half of a bivalve shell. Glass has
excellent conchoidal fracture.
Cretaceous—a division of geologic time, estimated to be the time from
135 to 60 million years ago; youngest division of the Mesozoic
era.
Crown—that portion of a faceted gem above the girdle; the upper
portion of a facet-cut gem (fig. 6).
Cryptocrystalline—composed of very fine or microscopic crystals.
Crystal—the regular polyhedral form, bounded by plane surfaces, that
is assumed by a mineral under suitable conditions. Crystals
have definite external symmetry and internal molecular order.
Crystalline—possessing definite internal molecular order; not
amorphous.
Cubic—in the general shape of a cube. The isometric crystal system is
often called the cubic system.
Culet—the very bottom portion of a faceted gem; the point or line
formed by the intersection of the lowest pavilion facets (fig.
6).
Dendritic—branching or tree-like in form.
Diaphaneity—relative transparency. The diaphaneity of a mineral is
described as transparent, translucent, opaque, etc.
Dike—a tabular rock body, usually igneous in origin, which cuts across
the surrounding rock strata.
Dispersion—a measure of the ability of gemstones to separate complex
or white light into its component colors; often illustrated
with a prism. Gemstones that are capable of separating colors
of light widely are said to have high dispersion; gemstones
not so capable of separating white light into colors are said
to have low dispersion.
Dopping—the act of cementing a gemstone, either rough or partly
finished, to a dop-stick.
Dop-stick—the wooden stick or cylindrical piece of metal to which a
gemstone is cemented to facilitate handling during cutting and
polishing.
Dop-wax—the agent or cement used to secure a gemstone to a dop-stick.
Emerald cut—a rectangular or square faceted stone with beveled corners
whose surfaces are covered with several series of rectangular
facets.
Eocene—a division of geologic time, estimated to be the time from 50
to 40 million years ago; one of the older divisions of the
Cenozoic era.
Extrusive rock—igneous rock that has been extruded or forced out onto
the earth’s surface.
Facet—a single plane polished surface on a faceted gem.
Facet head—a device used in the cutting and polishing of faceted gems;
used to control the placement of facets and their relative
angles (fig. 7).
Facet table—the equipment used in the cutting and polishing of faceted
gems and the table on which most of the equipment is mounted
(fig. 7).
Feldspar—a group of closely related silicate minerals including
orthoclase, microcline, sanidine, plagioclase, labradorite,
and others.
Fire—the reflections of variously colored light from a precious opal;
also the different colors of light reflected from a faceted
gem owing to the dispersion of the mineral.
Fracture—the texture of a freshly broken surface other than a cleavage
surface, described as conchoidal, even, splintery, etc.
Gem—a cut and polished gemstone.
Gemology—the science dealing with the study of gemstones.
Gemstone—a mineral suitable for cutting into a gem; the term gemstones
is frequently used collectively to include both cut and
polished stones and rough stones.
Geode—a rounded or spherical rock cavity; commonly lined with
crystals.
Girdle—the portion of a faceted gem separating the crown from the
pavilion; the girdle may or may not be polished and usually
contains about 2 percent of the total depth of the gem (fig.
6).
Gneiss—a coarse-grained metamorphic rock having segregations of
granular and platy minerals that give it a more or less banded
appearance without well-developed schistosity.
Grain (pearl grain)—a unit of weight equal to 0.05 gram or 0.25 carat;
not the same as the Troy grain.
Granite—a granular igneous rock composed mostly of quartz, feldspar,
and commonly mica and/or hornblende.
Hexagonal—having six angles and six sides; a crystal system in which
the crystal faces are referred to four intersecting axes;
three of these axes are equal, lie in the same plane, and
intersect at angles of 60 degrees; the fourth axis is
perpendicular to the other three.
Igneous rock—rock formed by solidification from a hot melt.
Index of refraction—a measure of the relative ability of a gemstone to
“bend” incident light rays; sine of the angle of incidence of
a light ray divided by the sine of the angle of refraction.
Intrusive rock—rock that has been pushed (usually in a molten state)
among pre-existing rock strata, commonly along faults or
fissures. Intrusive rocks do not reach the earth’s surface but
are commonly exposed at the surface by later erosion.
Isometric—a crystal system in which the crystal faces are referred to
three equal intersecting axes at right angles to each other.
Lap—a disc-shaped piece of metal or other material which is
impregnated with diamond dust, or some other cutting or
polishing agent, that is revolved while the gemstone is worked
against it.
Lap plate—a metal plate to which a cutting or polishing lap is
attached, usually by means of a threaded bolt and wing nut.
The lap plate is attached to the shaft which is turned by the
motor under the facet table.
Lapidary—one who practices the lapidary arts; a gem cutter.
Limestone—a sedimentary rock composed mostly of calcium carbonate.
Luster—the appearance of the freshly broken or unweathered surface of
a mineral in reflected light (p. 5).
Main facet—as applied to the standard American brilliant cut, one of
the first eight facets cut on either the crown or pavilion of
a gem (fig. 6).
Matrix—the material in which a specific mineral is embedded; also the
rock to which one end of a crystal is attached.
Metamorphic rock—rock that has been changed from its original state by
heat, pressure, chemical action, or some combination of these
factors.
Millimeter—¹/₁₀ centimeter; approximately ¹/₂₅ inch.
Mineralogy—the science concerned with the study of minerals, including
their occurrence, composition, forms, properties, and
structure.
Monoclinic—a crystal system in which the crystal faces are described
in relation to three intersecting unequal axes, two of which
are at right angles and the third inclined.
Oligocene—a division of geologic time, estimated to be the time from
40 to 28 million years ago; part of the Cenozoic era.
Opaque—does not transmit light.
Orbicular—containing orbs or spherical or eye-like markings or
structures.
Orthorhombic—a crystal system in which crystal faces are referred to
three unequal intersecting axes at right angles.
Pavilion—the portion of a faceted gem below the girdle (fig. 6).
Pegmatite—a body of coarse-grained intrusive igneous rock, commonly
lens or dike shaped.
Perthitic—a plaid-patterned structure resulting from intermixture of
soda- and potash-rich feldspars.
Phantom crystal—a crystal outline seen within another crystal, mostly
due to entrapping of inclusions during the crystal’s growth.
Pleochroism—the property of transmitting different colors of light in
different crystallographic directions.
Point—a unit of weight equal to ¹/₁₀₀ (0.01) carat.
Porous—containing pores or void spaces.
Precambrian—a division of geologic time, estimated to be all of
geologic time prior to 550 million years ago; the time before
the Paleozoic era.
Preform—a gemstone that has been ground to a rough outline of the
finished shape of a gem.
Rhyolite—a fine-grained extrusive or shallow intrusive igneous rock of
approximately the same composition as granite.
Rough—uncut, not worked by a lapidary, not cut and polished.
Schist—a metamorphic rock that contains an abundance of oriented platy
minerals that enable the rock to be split with relative ease
parallel to the flat surfaces of the platy minerals.
Silicified—replaced by or containing a large amount of quartz or
silica.
Skill facet—a term often used for the pavilion girdle facets of the
standard American brilliant cut (fig. 6).
Specific gravity—the weight in air divided by the loss of weight in
water at a given temperature, or the weight of an object in
air divided by the weight of an equal volume of water; also
called relative density; the most commonly used standard
temperature for this measurement is 4° C. or 39.2° F.
Star facet—one of the eight facets surrounding the table facet of a
standard American brilliant cut (fig. 6).
Step cut—a mode of faceting in which the surface of the gem is covered
by a series of square or rectangular facets; stones thusly cut
are usually square, rectangular, or irregular with straight
sides in outline.
Streak—the color of a mineral when finely powdered; usually determined
by rubbing the mineral against a piece of unglazed porcelain.
Symmetry—the number, location, and balanced arrangement of crystal
faces in reference to the crystallographic axes or other
crystallographic planes or directions.
Synthetic gem—a gemstone manufactured by man that has approximately
the same chemical composition and properties as a natural
gemstone.
Table facet—the large horizontal facet found on the crown of many
gems, often called simply the table (fig. 6).
Tenacity—the resistance of minerals to breakage, described by such
terms as malleable, ductile, sectile, and brittle (p. 6).
Termination—the end of a crystal that is completely enclosed by
crystal faces, the crystal end that is not attached to the
matrix.
Tertiary—a division of geologic time, estimated to be the time from 60
to 1 million years ago; the Tertiary includes the Paleocene,
Eocene, Oligocene, Miocene, and Pliocene epochs (from oldest
to youngest).
Tetragonal—having four angles; a crystal system in which the crystal
faces are referred to three axes at right angles to each
other, two of which are equal and the third longer or shorter.
Translucent—allowing the passage of light but diffusing it
sufficiently so that objects on the other side cannot be
clearly distinguished.
Transparent—clear, allowing free passage of light so that objects on
the other side can be readily distinguished; opposite of
opaque.
Triclinic—a crystal system in which the crystal faces are referred to
three unequal axes, none of which are at right angles.
Tumbling—a process of polishing irregularly shaped gemstones (p. 17).
Vein—a tabular, irregular, or twisting mineral deposit that is thin in
relation to its length and breadth, usually the result of
solution or hydrothermal activity.
Vitreous—having luster, general appearance, or physical properties
similar to glass.
Vug—an unfilled rock cavity, commonly lined with crystals; may later
become filled by minerals owing to solution or hydrothermal
activity.

SELECTED REFERENCES

Anderson, B. W. (1948) Gem testing: Emerson, New York.

Baker, C. L. (1935) Metallic and non-metallic minerals and ores (precious stones), _in_ The geology of Texas, Vol. II, Structural and economic geology: Univ. Texas Bull. 3401, Jan. 1, 1934, pp. 568-569.

Barnes, V. E. (1940) North American tektites: Univ. Texas Pub. 3945, Dec. 1, 1939, pp. 477-582.

Dake, H. C., Fleener, F. L., and Wilson, B. H. (1938) Quartz family minerals: Whittlesey House, McGraw-Hill Book Company, Inc., New York.

Ford, W. E. (1932) A textbook of mineralogy (4th ed.): John Wiley and Sons, Inc., New York.

Kraus, E. H., and Slawson, C. B. (1947) Gems and gem materials (5th ed.): McGraw-Hill Book Company, Inc., New York.

Kunz, G. F. (1892) Gems and precious stones of North America (2d ed.): Scientific Publishing Company, New York.

Pough, F. H. (1953) A field guide to rocks and minerals: Houghton Mifflin Company, Boston.

Simpson, B. W. (1958) Gem trails of Texas: Granbury, Texas.

Sinkankas, John (1955) Gem cutting: D. Van Nostrand Company, Inc., Princeton, New Jersey.

—— (1959) Gemstones of North America: D. Van Nostrand Company, Inc., Princeton, New Jersey.

Smith, G. F. H. (1958) Gemstones (13th ed.), revised by F. C. Phillips: Methuen and Company, Ltd., London.

Sperisen, F. J. (1950) The art of the lapidary: The Bruce Publishing Company, Milwaukee, Wisconsin.

Sterrett, D. B. (1913) Gems and precious stones, _in_ Mineral resources of the United States, Calendar Year 1912, Part II, Non-metals: U. S. Geol. Survey, pp. 1023-1060.

Plate I

Plate II

Plate III

Plate IV

Plate V

Plate VI

Index

A
actinolite: 26
agate: 20, 28, 38
agatized wood: 27
allanite: 21
almandite: 22
amazonite: 23
amazon stone: 23
amber: 18
amethyst: 25
Amethyst Hill: 25
amorphous gemstones: 9
andalusite: 30
Arkansas: 19
Armstrong County: 21
augite: 18, 28

B
Baringer Hill, Llano County: 21, 26
baroque pearls and/or stones: 17, 25
bediasite (tektite): 28-29, 39
beryl: 18
Big Bend National Park: 27
biotite: 23
Blanco County: 18, 22
Brazos County: 29
Brazos River: 25
Brewster County: 18, 23, 24, 25, 26, 27, 28, 31, 36
brilliancy: 5
brilliant cut, standard American: 13, 15, 16
Brown County: 19
Burleson County: 29
Burnet County: 20, 22, 23, 25, 26, 30, 37