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• Physical traits and mineral identification
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Minerals are the natural, inorganic (nonliving) materials that compose rocks. Examples are gems and metals. Minerals have a fixed chemical makeup and a definite crystal structure (its atoms are arranged in orderly patterns). Therefore, a sample of a particular mineral will have essentially the same composition no matter where it is from—Earth, the Moon, or beyond. Properties such as crystal shape, color, hardness, density, and luster distinguish minerals from each other. The study of the distribution, identification, and properties of minerals is called mineralogy.

Almost 4,000 different minerals are known, with several dozen new minerals identified each year. However, only 20 or so minerals compose the bulk of...