A clear, commonly rounded, variety of [opal] that looks like ice is called [hyalite]. Two areas in which it has been found are in Presidio County in west Texas and in Llano County in central Texas.
Opalized wood from Washington County, Texas.
A variety of petrified wood, called opalized wood, is [opal] that replaced the fibers of a piece of wood. [Wood opal] is found at a number of places in the [Gulf Coastal Plain]. It occurs there in [Tertiary] [formations] within about 20 miles of the boundary line between areas 2 and 3 shown on the [geologic map] (pp. [4]-5).
A soft opaline material called [diatomite], or diatomaceous earth, is made up chiefly of the skeletons of diatoms—tiny, one-celled plants that live in fresh or [salt] water. These little plants are able to take silica from the water to make [opal] skeletons for themselves. When the diatom skeletons collect at the bottom of a lake or sea, they form the light, crumbly, white, gray, or cream-colored deposit of impure opal known as diatomite. Industry uses this material as a filter, as insulation, as an abrasive, and as a filler.
[Diatomite] formed in ancient lakes on the Texas [High Plains] during late [Tertiary] ([Pliocene]) and early [Quaternary] ([Pleistocene]) times. It is found in Armstrong, Crosby, Dickens, Ector, Hartley, and Lamb counties.
Opaline Granite. See [Llanite].
Orthoclase. See [Feldspar].
Pegmatite
Pegmatites occur in [igneous] rock areas, and most geologists consider them [intrusive] [igneous rocks]. They are made up of crystals and [crystalline] mineral grains that fit together—the grains are interlocked. The crystals and grains in pegmatites are larger than those of surrounding rocks, and some are huge, even larger than a man. However, there is a wide range of grain sizes in [pegmatite].