Minerals are naturally occurring, crystalline, inorganic, homogeneous solids with chemical compositions that are either fixed or vary within certain fixed limits. They have a characteristic internal structure which is manifested in their external form and physical properties.

Common minerals are identified or recognized by specific physical properties. For example, the common substance table salt is actually a mineral composed of sodium chloride (NACI) and bears the name halite. The taste of halite is distinctive and is sufficient for identifying and distinguishing the mineral from other substances. Other minerals may be identified by their color (sulfur), hardness (diamond), crystal form (quartz) or cleavage (the tendency of some minerals to break apart along definite planes of weakness that exist in their atomic structure - fluorite). Some minerals are magnetic (magnetite) or react to acid (calcite), some have an unusual luster (talc), a characteristic specific gravity (pyrite) or can be distinguished by diaphaneity (the ability to transmit light) or tenacity (muscovite and biotite).

Two or more crystals of some minerals may be grown together in such a way that the individual parts are related through their internal (atomic) structure. The external form that results is manifested in a twinned crystal. Many minerals such as plagioclase, fluorite, orthoclase and staurohte have such characteristic twinning habits that they are easily identified.

There are over 1,200 known minerals in nature, many are quite rare and are known to exist in only one area. Most of the minerals in the Department of Natural Science's collection are common minerals, but minerals which have grown to uncommon size, or display a particular physical property uncommonly well.

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