The Indians who long ago roamed this area used the smooth, shiny [vitrophyre] and [obsidian] to make some of their arrowheads and scrapers. Today, rock collectors pick up these attractive rocks for their collections, and some of them cut and polish obsidian and vitrophyre for use as gemstones.
Onyx. See [Quartz].
Öolitic Limestone. See [Limestone].
Opal
[Opal] is like hardened jelly or gelatin. It has no [crystalline] inner structure and no crystal shape of its own—it is [amorphous]. This mineral has almost the same chemical composition as [quartz]. Both are silicon dioxides (silica), but opal, in addition, contains water.
[Opal] can be almost any color—red, yellow, blue, brown, gray, white—or it can be colorless. It is [transparent] or [translucent] and appears glassy, resinous, greasy, or dull. Opal has a [specific gravity] of 1.9 to 2.2—this mineral is a little lighter than [quartz]. It also is softer than quartz. A copper penny will not scratch opal, but quartz will. Opal has a white [streak] and a curved, [conchoidal] [fracture] but no [cleavage].
[Opal] occurs in a number of places in Texas. In the Trans-Pecos country of west Texas, it fills cracks and cavities in some of the [extrusive] [igneous] rocks. It occurs on the [High Plains] of northwest Texas, and it is found in [Tertiary] [formations] of the [Gulf Coastal Plain] where it occurs as masses that fill cracks and cavities in [sedimentary rocks], as the cementing material in some sandstones (such as in the Catahoula [sandstone]), and as opalized wood.
Much [opal] forms from underground waters that contain silicon. These solutions move through the rocks and deposit the opal in them.
[Opal] is found in a number of varieties. Some show a beautiful, lustrous play of colors that comes from inside the specimens. These varieties are known as [precious opal] and are prized as gemstones. In Texas, some precious opal is found near Alpine in Brewster County. It has a milky white to bluish-white color, is [translucent], and shows a fiery orange, red, blue, and green play of colors.
The variety known as [common opal] shows no play of colors. It may be white, gray, bluish, reddish, greenish, or yellowish, and it is only slightly [translucent]. It is found in Brewster, Jeff Davis, Presidio, and other counties of the Trans-Pecos country of west Texas. It occurs also around some of the wet-weather ([playa]) lakes on the Texas [High Plains]. In the [Gulf Coastal Plain], common opal is found with [chalcedony] (a variety of [quartz]) in [Tertiary] [formations]. A south Texas locality sometimes visited by collectors is near Freer in Duval County.