In this connection it is interesting to note that the polish on limestone or marble is not durable where exposed to the weather in the same sense as is the polish on granite. Because limestone and marble are softer than granite they may be cut and polished at lower cost, but because of their ease of attack by acid, water, and abrasion they soon become dull when used as an exterior stone. For interior decoration they are excellent, of course. Granite contains hard minerals which happen not to be attacked appreciably by dilute acids, and therefore it retains a polish for a long time even where exposed to the weather.
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 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 (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.