_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.