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. [Illustration: Fig. 16. Crystal faces on microcline specimen shown in Plate III, A.] 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