Muscovite. _See_ Mica. Native Silver. _See_ Silver Minerals. Obsidian and Vitrophyre Obsidian is a dark, glassy-looking igneous rock. Most obsidian contains the same chemical elements as granite and rhyolite, since all three of these rocks can form from the same type of molten rock material. Obsidian, however, has no separate minerals, because its chemical elements are not combined in an orderly way. It is a natural glass. Because it is a glass, we know that obsidian forms very quickly. One way for it to form is from the sudden cooling of hot, molten lava that flows out of volcanoes. If the lava cools and hardens before the separate minerals can crystallize, it becomes a natural glass, such as obsidian. This rock is smooth and shiny. Most of it is black, but some can be dark green or dark brown. Obsidian allows light to pass through it, and it breaks with a curved, conchoidal fracture. The broken edges are very sharp. Another glassy igneous rock that forms from fast-cooling lava is _vitrophyre_. It looks like obsidian except that it has crystals or crystalline mineral grains (which may be light colored) scattered through the dark glassy material. [Illustration: Obsidian was used by the Indians to make arrowheads.] Obsidian and vitrophyre are found in the Big Bend area of Brewster and Presidio counties in west Texas. They occur with other igneous rocks that formed there during Tertiary time. 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.