Part 6

Microcline

Composition: KAlSi₃O₈. Crystal system: triclinic. Hardness: 6.0 to 6.5. Specific gravity: 2.54 to 2.57. Luster: vitreous to pearly. Color: white, pale yellow, red, blue green, bluish. Streak: white. Cleavage: four directions, usually three of these distinct. Fracture: uneven. Tenacity: brittle Diaphaneity: transparent to translucent. Refractive index: about 1.52 to 1.53.

Very fine crystals of blue microcline have been found east of Packsaddle Mountain and near Kingsland in Llano County. Crystals exceeding 1 foot in length have been found, although most are only a few inches long. The color of the microcline is mostly pale blue, but some crystals are darker. Microcline crystals associated with milky or vein quartz, smoky quartz, some biotite, and rarely cassiterite occur in pegmatite dikes which vary in size from a few inches to several feet in thickness. The color of this microcline is pale in comparison to microcline from some other localities in the United States, but the Texas blue microcline does yield pleasing cabochons. Perfect crystals of this material are prized by collectors. Blue or greenish microcline is often called amazonite or amazon stone.

Bluish microcline associated with quartz and topaz has also been reported near Katemcy, Mason County.

Red microcline is common in several central Texas counties and is a primary constituent of many of the igneous rocks in those counties. Large crystals of perthitic red microcline occur in pegmatite dikes of Mason, Llano, Burnet, and Gillespie counties. Any feldspar quarry or other pegmatite mining operation in any of these counties is likely to contain large red microcline crystals and fragments. Unfortunately, the good crystals that may have been present are often shattered by blasting during quarrying operations.

Feldspar quarries in northeastern Gillespie County have yielded some good red cabochon material as well as good crystals. Here the microcline occurs with milky and smoky vein quartz, smoky quartz crystals, clear quartz crystals, greenish muscovite, and biotite. Many of the older quarries in Gillespie County have not been active for some time, and the dumps and quarry walls have been diligently searched by collectors.

Many of the pegmatite dikes near Lake Buchanan in Llano and Burnet counties have produced some good red microcline specimens and cutting material (Pl. III, A, and fig. 16). Many of these crystals are more pinkish than those in Gillespie County, but this is commonly due to the fact that the crystal faces of the Lake Buchanan area crystals are somewhat more weathered than the fresh Gillespie County crystals.

Numerous other local areas in the counties mentioned, as well as some localities in Hudspeth and Culberson counties, have also produced small amounts of red and pink microcline of gem quality.

Obsidian

Composition: volcanic glass. Structure: amorphous. Hardness: 5.0 to 5.5. Specific gravity: 2.3 to 2.5. Luster: vitreous. Color: black, dark gray, reddish, brown, bluish, and greenish. Streak: white. Cleavage: none. Fracture: conchoidal. Tenacity: brittle. Diaphaneity: translucent to nearly opaque. Refractive index: variable, about 1.45 to 1.53.

Gem-quality black and dark-gray obsidian has been found in Presidio County associated with extrusive igneous rocks. The obsidian in this area is too opaque to serve as attractive faceted stones but is found in pieces of sufficient size and quality to yield nice cabochons. Some of the small weathered pieces of this material resemble tektite in outward appearance; in fact, the “valverdites” mistaken originally for tektites are pebbles of weathered obsidian in terrace gravel of Val Verde County. Obsidian takes a high polish but is very sensitive to heat. Stones that are slightly overheated during grinding or sanding will quickly shatter.

Obsidian of gem quality has been reported also in Brewster County.

Opal

Composition: SiO₂·nH₂O. Structure: amorphous. Hardness: 5.5 to 6.5. Specific gravity: 1.9 to 2.3. Luster: subvitreous to pearly. Color: white, bluish, pink, brown, yellow, and gray. Streak: white. Cleavage: none. Fracture: conchoidal. Tenacity: brittle. Diaphaneity: transparent to nearly opaque. Refractive index: 1.43.

Opal other than as fossil or opalized wood (pp. 20-21) occurs at the following several localities in Texas.

Approximately 16 miles south of Alpine, Brewster County, precious opal occurs in very small seams and as cavity fillings in very hard pinkish-brown rhyolite. This opal is milky or bluish and commonly exhibits small flashes of blue, green, red, and orange fire. Individual pieces of this opal are mostly quite small, rarely over one-fourth inch in diameter, and very difficult to remove from the tough rhyolite matrix. Local lapidaries have cut interesting cabochons from this material in which several small patches of opal that are close together in the matrix are included in the same cabochon.

Small finds of opal associated with rhyolites and basalts have come from other localities in west Texas, but the opal mostly does not display enough play of colors to warrant its use as gem material.

Near Freer, Duval County, some very attractive common opal has been found. The opal is colored various shades of pink, blue, and yellow and in certain local areas occurs as fragments that are cemented together by clear chalcedony. Various colors are commonly found in the same piece, and such material yields handsome cabochons. Although the area has never been worked commercially, it has been hunted by collectors and cutters for several years.

Pearl

Pearls are the result of the secretion of calcium carbonate by various shellfish around sand grains, parasitic organisms, shell fragments, or other foreign objects that have in some way entered the body cavity of the shellfish. Since the shellfish is unable to expel these irritating particles or organisms, it deposits successive layers of calcium carbonate around the foreign substance to make it smoother and less irritating. Although pearls are principally calcium carbonate, they also contain small amounts of an organic substance, called conchiolin, and water. Pearls are found in shellfish that live in either fresh or salt water. Few pearls are spherical in shape; most are rounded but somewhat irregular and are known as baroque pearls. Good quality pearls are the only gemstone commonly sold by the grain, a unit of weight equal to 0.25 carat or 0.05 gram. The pearl grain is not the same unit of weight as the Troy grain.

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.