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.
Good groups of pale amethyst crystals have been found in quartz veins near the old town site of Oxford, Llano County. The occurrence seems to be much the same as the Amethyst Hill locality. Little exploration for gemstones has been done in this area, and future discoveries seem likely.
Chalcedony geodes lined with amethyst crystals have been found in Brewster, Presidio, Culberson, and Hudspeth counties, but the occurrences are scattered. The crystals are seldom large enough to yield gems of more than 3 carats and are mostly very light colored.
A few pieces of gem-quality amethyst have been found in Burnet County.
Citrine (yellow quartz).—Very little gem-quality citrine has been reported in Texas. Some small citrine crystals have been found at Amethyst Hill in northeastern Gillespie County, but few are of sufficient size or color to yield good gems.
The writer has seen one citrine crystal that was found in the gravels of a small stream in eastern Llano County near Buchanan Dam. The crystal weighs about 1 ounce and is perfectly clear, light golden yellow, and flawless. However, a further search of the stream gravels failed to produce any other citrines.
Rock crystal (colorless quartz).—Numerous localities in Texas produce this colorless variety of quartz, which is the most common variety of facet quality quartz and consequently is of little value.