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. 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. [Illustration: 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.