Sometimes, prospectors use a chemical test to help them identify cassiterite. They put small pieces of metallic zinc into a jar or test tube containing dilute hydrochloric acid. Then they add a few fragments of the mineral that they suspect is cassiterite. If the fragments are cassiterite, they become covered with a pale gray coating of metallic tin. Cassiterite’s most common crystal shape is a short, 8-sided prism with pyramids at each end, but perfect crystals are not often found. Most Texas cassiterite does not show a crystal shape. Instead, it occurs as crystalline masses in igneous rocks and as loose pebbles that have weathered out of these rocks. Cassiterite occurs in a number of places in the United States but not in large quantities. A small amount of cassiterite has been found in quartz veins in Precambrian granite in both central Texas and west Texas. In El Paso County, the cassiterite is found on the east side of the Franklin Mountains a few miles north of El Paso, where some of it has been mined. In central Texas, cassiterite occurs in the Streeter area of Mason County. When the granite rocks in these areas were formed, probably not all of the hot magmas cooled and hardened at the same time. The fluids given off by the remaining magmas contained tin and several other elements. It is believed that these fluids moved up into cracks in the granite rocks and formed the cassiterite. Celestite Celestite is a strontium sulfate mineral. It is colorless, white, yellow, or gray. Light blue specimens of this mineral also are found, and it is because of this sky-like color that celestite gets its name. The word celestite comes from the Latin word _caelestis_, meaning _of the sky_. Celestite has a glassy to a pearly luster, and it is either transparent or translucent. It gives a white streak when rubbed across a streak plate. Celestite has a specific gravity of 3.95 to 3.97. It is, however, lighter than barite, a mineral that it resembles. Celestite is not very hard—a knife will scratch it, although your fingernail will not. It cleaves in three directions, and some of the fragments are flat and slabby. [Illustration: Celestite cleavage fragment from Lampasas County, Texas.] Celestite occurs commonly either as prism-shaped or flat crystals and as cleavable, granular, or fibrous crystalline masses. In Texas, it is found in geodes, as rounded nodules, or as bedded or layer-like deposits in limestones and other sedimentary rocks. In Real County, celestite occurs on the walls of a cave in Cretaceous limestone. Some celestite may be deposited by sea water, but much of the Texas celestite is believed to have been deposited by underground water that seeped through cracks and pores in the limestones and other sedimentary rocks. This water picked up and dissolved strontium compounds that were scattered in small amounts through the rocks. Then, it re-deposited the strontium in the rocks as celestite.