|MadSci Network: Earth Sciences|
Tough question that really does not have a simple answer. In real life, experience and training are the best tools for recognizing what kind of rock you have. After working with different kinds of rocks, you just get to "know" what kind they are; after seeing and handling many different kinds of limestone, for example, you get to be able to recognize another piece of limestone when you see it. Some information comes from the provinence of the rock, that is, where did it come from. If a rock came from the side of a volcano, it would probably be igneous, for example. Sometimes, there are internal clues. For example, fossils indicate sedimentary rocks (or sometimes metamorphic rocks that come from sedimentary rocks) because igneous processes are so hot they burn up the material to be fossilized. Usually, rocks with parellel planes of rock are sedimentary. Sometimes microscopic examination of a rock can tell something about its origins, for example, a rock made of fine crystals could be igneous (granite) or metamorphic (gneiss) or sedimentary (conglomerate or sandstone). The crystals in granite would be jagged at the edges, the crystals in gneiss would be squished and deformed by pressure and heat, and the crystals in sandstone would likely be rounded on the edges by erosion before the sand was deposited. Rocks are composed of minerals, of course. Trained rock-hounds (or petrologists) learn to recognized the important minerals that make up rocks. Then, by knowing where and how different minerals are made, they can often tell much about the rock that contains them. There are other, more complicated chemical tests that can be done but they are not usually available when you first look at a rock.
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