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.
Fig. 17. Common crystal form of spinel.