Transparent crystals and cleavage fragments of calcite show a property called _double refraction_ (other minerals show it, too). To test this property, you can mark a single dot on paper. When you look at the dot through a piece of clear calcite, you will see two dots instead of one. This happens because a ray of light is bent (refracted) and is split into two rays as it enters the mineral. These two rays travel through the calcite in slightly different directions, and each carries an image of the dot through the mineral. The two images that you see are at the points where the two rays leave the calcite. Calcite that is deposited at springs, along river and creek banks, and in caves is known as _travertine_. Cave forms of travertine, including stalagmites and stalactites, occur in several caves in Texas. Another kind of travertine is called _calcareous tufa_ or _calcareous sinter_. It is a porous and spongy-looking material deposited from water carrying dissolved limestone and is found around the openings of some springs and along some creek and river banks. A dull, earthy calcite deposit, known as _caliche_, occurs in areas of Texas that have scant rainfall, such as the High Plains, west Texas, and the southwestern part of the Gulf Coastal Plain. Caliche commonly is found mixed with other materials, such as clay, sand, or gravel. This substance may be firm and compact or loose and powdery. It is thought that caliche forms when ground moisture, containing dissolved calcium bicarbonate, moves upward. In dry areas of the country, this moisture evaporates. As it does, it leaves a crust of calcium carbonate in the form of caliche on or near the surface of the ground. [Illustration: Calcite crystals (dog-tooth spar) from the Terlingua area of Brewster County, Texas.] Caliche is quarried in many counties in Texas and is used chiefly as road material and as an aggregate. Caliche. _See_ Calcite. Carnotite. _See_ Uranium Minerals. Cassiterite Cassiterite, tin dioxide, is the mineral that serves as the chief source of tin. Tin does not corrode and tarnish, and one of its main uses is in the making of tin cans. (Actually, our tin cans are made from thin sheets of steel that have been coated with a protective layer of tin.) Cassiterite has either a nonmetallic or a submetallic luster. Some specimens are brilliant and shiny; others are dull. Cassiterite may be translucent to transparent. It may be black, brown, gray, reddish brown, or yellowish brown. When rubbed across a streak plate, this mineral leaves a pale brown, a pale yellow, or a white streak. Cassiterite is quite heavy—it has a specific gravity of 6.8 to 7.1. It is too hard to be scratched by an average pocket knife.