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. The amethyst tends to be very irregularly colored in zones parallel to the crystal faces. In many, the base of the crystal is colorless or white and only the termination is violet. Crystals up to 3 inches long have been found at this locality, but the average size is much less. The surface at this locality is almost entirely depleted of amethyst, with only an occasional small crystal or fragment to be seen. However, small excavations are still sometimes productive.