[Amethyst] geode from the Alpine area of Brewster County, Texas.
A colorless, glassy variety of [quartz], called [rock crystal], is clear enough to see through. It is found as crystals that are 6-sided prisms with pyramid-like faces on the ends. This variety is commonly associated with [igneous rocks], such as those of the [Llano uplift] area of central Texas and of the Trans-Pecos country of west Texas. It is commonly used as a gemstone and is made into necklaces, earrings, and other jewelry. Some specimens of rock crystal have slender, needle-like crystals of other minerals, such as [tourmaline], actinolite, or rutile, enclosed in them.
A clear, glassy variety of [quartz], [amethyst], has a purple or violet color. It, like [rock crystal], is commonly found in 6-sided prisms with pyramid-shaped ends and is also prized as a gemstone. Amethyst has been found in [Precambrian] rocks in the [Llano uplift] area of central Texas. (Amethyst Hill, a locality well known to collectors for many years, is in northeastern Gillespie County.) In west Texas, amethyst has been found in [Cenozoic] [igneous] rocks in the Sierra Blanca and Quitman Mountains of Hudspeth County and in the Alpine area of Brewster County.
[Milky quartz] from Burnet County, Texas.
A variety of [quartz] with a milk-white color and a glassy to greasy luster is called [milky quartz]. It occurs either as crystals or as [crystalline] masses. Very little light will pass through it. In central Texas, milky quartz occurs abundantly in the [Precambrian] rocks of the [Llano uplift] area in Blanco, Burnet, Gillespie, Llano, and Mason counties. It also is found in some of the rocks of the Trans-Pecos country of west Texas, such as in the Carrizo Mountains of Culberson and Hudspeth counties. Other good places to look for this variety of quartz are in the sands and [gravels] along many streams in Texas.
Some [quartz] has a glassy to a greasy luster and a rose or pink color. [Rose quartz], as this variety is called, commonly occurs as masses rather than as individual crystals. It can be found along some of the streams in Texas and also in [igneous rocks], such as those of the [Llano uplift] area of central Texas.
A kind of [quartz] with a smoky brown, a smoky yellow, or a dark brownish-black color is called [smoky quartz]. Its luster is glassy, and it may be either [translucent] or [transparent]. Smoky quartz is commonly found as crystals that are shaped like 6-sided prisms with pyramid-like ends. It is commonly associated with [igneous rocks], and beautiful specimens have been found in the Lake Buchanan area of Llano and Burnet counties in central Texas.
Smoky-quartz crystals from Burnet County, Texas.