Part 4

Specimens and pieces of cutting quality 1 inch in diameter are common, and fragments over 2 inches in diameter have been found. The augite is associated with black spinel and some dark gray to black pieces of natural glass. Although the faceted and cabochon-cut stones are not particularly attractive, some of the larger pieces of augite might be utilized for carving.

Beryl

Composition: Be₃Al₂(SiO)₆. Crystal system: hexagonal. Hardness: 7.5 to 8.0. Specific gravity: 2.63 to 2.80. Luster: vitreous. Color: pale blue, blue, green, yellow, brownish, pink, and colorless. Streak: white. Cleavage: one direction, very imperfect. Fracture: conchoidal to uneven. Tenacity: brittle. Diaphaneity: transparent to subtranslucent. Refractive index: 1.56 to 1.60. Dispersion: low.

Gem-quality beryl has not been reported in Texas. A discussion of beryl is included herein because the writer believes it likely that beryl of gem quality will be found in Texas as a result of future investigations and exploration.

Beryl crystals have been found in pegmatite dikes in Llano, Blanco, and Gillespie counties. These crystals are commonly several inches long and exceed 1 inch in diameter but are very badly fractured. Most of the beryl crystals do not approach gem quality and are entirely unsuitable for any lapidary use. The color of the crystals found thus far is bluish, greenish, pinkish brown, yellowish, and colorless. Some very tiny colorless beryl crystals have been found that are transparent, but thus far such crystals have been too small to be cut into gems.

Fine blue beryl crystals have been found in the Franklin Mountains near El Paso, Texas. Unfortunately, these crystals are so badly flawed and fractured that they are not suitable for lapidary use.

It seems likely that careful prospecting of Texas pegmatites will reveal at least some gem-quality beryl.

Celestite

Composition: SrSO₄. Crystal system: orthorhombic. Hardness: 3.0 to 3.5. Specific gravity: 3.95 to 3.98. Luster: vitreous. Color: white, blue, greenish, reddish, and brownish. Streak: white. Cleavage: three directions, although one of these directions is not easily developed. Fracture: uneven. Tenacity: brittle. Diaphaneity: transparent to subtranslucent. Refractive index: 1.62 to 1.63. Dispersion: moderate.

Celestite is very seldom cut into gems. Being very soft, brittle, and having three cleavages, celestite is completely unsuitable for jewelry. These same properties make this mineral exceedingly difficult to facet; however, faceted stones are seen in large collections.

Fine crystals of colorless and blue gem-quality celestite (Pl. I, A, and fig. 14) have been found at Mount Bonnell and other localities west of Austin, Travis County. The celestite crystals occur in vugs or geodes in limestone. The crystals are mostly white or colorless and fractured near the base or where attached, but the tips of the crystals are commonly clear celestine blue and completely free of flaws.

Crystals several inches in length have been found, but the average size is about 1 inch. The smaller crystals are frequently more transparent and consequently better suited for cutting. It is very difficult to obtain crystals that will allow the cutting of flawless stones of more than 4 or 5 carats.

Bluish and colorless celestite of gem quality and fine crystals have been found near Lampasas, Lampasas County, and near Brownwood, Brown County, but neither of these localities has been very productive of good gem material.

Celestite geodes have been found in parts of Coke, Fisher, and Nolan counties, but these geodes contain little gem material.

Diamond

Composition: carbon. Crystal system: isometric. Hardness: 10. Specific gravity: 3.51 to 3.53. Luster: adamantine to greasy. Color: brown, colorless, pink, blue, yellow, and various other light colors; rarely deeply colored; sometimes black. Cleavage: four directions, octahedral, perfect. Fracture: conchoidal. Tenacity: brittle. Diaphaneity: transparent to opaque. Refractive index: 2.42. Dispersion: high.

There is only one well-authenticated find of diamond in Texas. A small brownish diamond was found in 1911 on section 64, block 44, Foard County (Sterrett, 1912, pp. 1040-1041). The exact weight of the stone has not been recorded, but one authority estimated that it was of sufficient size and clarity to yield a cut stone of about one-quarter carat.

The only diamond-bearing rocks known in the United States are in Pike County, Arkansas. Although many other diamonds have been found in the United States, all were loose in gravels or streams except for some stones at the Arkansas locality. The fact that one diamond was found in Foard County does not mean that the prospects of finding more diamonds in Texas are much better there than anywhere else in the State. It is highly unlikely that more than a very few diamonds will ever be found in Texas, and any stones that may be found in the future are likely to be widely scattered.

Epidote

Composition: HCa₂(Al, Fe)₂Si₃O₁₃. Crystal system: monoclinic. Hardness: 6 to 7. Specific gravity: 3.25 to 3.5. Luster: vitreous. Color: yellowish green to brownish green and brown. Streak: uncolored to grayish. Cleavage: two directions. Fracture: uneven. Tenacity: brittle. Diaphaneity: transparent to opaque. Refractive index: about 1.72 to 1.77.

Llano County has furnished some green and brownish-green epidote that is suitable for cutting into cabochons. Most of the material that approaches gem quality has come from contact metamorphic zones and is associated with garnet, quartz, and some scheelite. Some small cavities in the rocks contain tiny transparent crystals of gem quality, but the largest obtainable flawless faceted stones would probably be less than 15 points.

Faceted stones of epidote are sometimes known as pistacite owing to their common pistachio-green color.

Fluorite

Composition: CaF₂. Crystal system: isometric. Hardness: 4. Specific gravity: 3.0 to 3.25. Luster: vitreous. Color: violet, blue, colorless, green, yellow, brown, rose, and crimson red. Streak: white. Cleavage: four directions, octahedral, perfect. Fracture: subconchoidal to splintery. Tenacity: brittle. Diaphaneity: transparent to subtranslucent. Refractive index: 1.434.

Very fine green, transparent fluorite has been found near Voca, Mason County. The fluorite occurs as vug fillings in pegmatites, associated with crystals of pink microcline and colorless quartz. Most of the vugs have been completely filled by the fluorite; therefore, crystals (fig. 15) of the fluorite are not too common. Masses of fluorite several pounds in weight, rich green, and quite transparent have been found near Voca. Transparent pieces an inch or more in diameter are common.

Fluorite is much too soft for everyday use in jewelry and because of the low refractive index does not yield brilliant faceted stones. The perfect four-directional cleavage, relative softness, and brittle tenacity of the mineral make it difficult to facet. Faceted stones are seldom seen outside of collections. Cabochons are also difficult to cut from this material, but the rich color obtained is ample reward for the time and care necessary in cutting.

Fluorite occurs at several other localities in Texas, notably in Hudspeth, Brewster, Presidio, Llano, and Burnet counties, but not commonly in gem quality or colors that warrant its use as gem material.

Fossil Wood