The stalactites (rock icicles) hanging from cave ceilings, stalagmites built up from the floors, and other drip stone deposits of caves are largely calcite, the mineral of limestone. Again, this can be recognized by the limestone acid test (effervescence, see limestone). Cave onyx may be banded like agate. It is then commonly called Mexican onyx. The name travertine has also been applied to such deposits from water. Travertine Travertine is a general name for calcium carbonate deposits of varying size, shape, color, texture, and purity which originate largely through evaporation of spring or surface water. Its composition of calcium carbonate, calcite mineral, is easily confirmed by effervescence in acid, like limestone. Calcite Calcite (sometimes called “tiff” locally in south-_western_ Missouri), the essential mineral in limestone, can be recognized by several definite characteristics: 1. It bubbles, “fizzes,” or _effervesces_ in dilute acid. See page 11. 2. It is easily scratched with a knife. 3. It breaks or cleaves into rhombohedral shapes, of which at least one flat, glistening side is visible on every individual grain in the broken surface of limestone. 4. It has a glassy luster on crystal and cleavage faces. 5. It crystallizes into six-sided crystal forms, which can be differentiated from quartz (also six-sided) by tests (1) and (2) above. The one single test of calcite which is most diagnostic, and which appeals to most persons, is number one above, effervescence of the solid lump in dilute acid. The bubbles are filled by carbon dioxide gas which comes from, and is freed from, the calcite by the reaction of it with the acid. Calcite is calcium carbonate, CaCO₃. [Illustration: A small calcite crystal from the Joplin region.] Many Missourians have not realized that the ordinary, everyday limestone (fine to coarse granular), which is so abundant here, is composed of a mineral—calcite which makes up the grains. The strikingly beautiful calcite crystals (displayed in museums) derived from the calcite crystal caves found in some mines in the Joplin district are accepted without question as _mineral_ specimens of calcite, but the idea that all of the commonplace glistening grains in the local limestone are also mineral grains is a new thought to most persons. A pure limestone is composed entirely of calcite. Even impure limestones which contain subordinate amounts of quartz sand, chert, clay, or iron oxide are in the main also calcite. Dolomite and dolomitic limestones contain the mineral dolomite.