In general, we may think of minerals as the materials of which the rocks of the earth's crust are made, and, as such, minerals constitute the most important physical and tangible link with the earth's history. The mineralogist regards the minerals he finds in rocks and veins as the stable end products of a complex of natural processes which it is his task to unravel. This task he approaches by a study of the characters of the individual mineral and by consideration of the relations in time and space, or paragenesis as it is called, of the different mineral species constituting a rock or mineral deposits. The great majority of rocks consist of a more or less heterogeneous aggregate of several different minerals, but a few like limestone and quatrzite are made up of essentially one mineral in a more or less pure state. Although the great bulk of minerals occur as essential and integral constituents of rocks, others, frequently of greater significance to the studies of the mineralogist, are in veins and cavities, this mode of occurrence being typical of many of the economically valuable minerals or ores, from which the metals of commerce are obtained. Because the primary goal of the mineralogist is the elucidation of the physical, chemical, and historical aspects of the earth's crust, the term mineral and the study of mineralogy are limited to materials of natural occurrence. Thus steel, cement, plaster, and glass, although all are derived from naturally occurring mineral raw materials, are not regarded as minerals themselves, since they have been processed by man. A synthetic ruby, although it is chemically, structurally, and physically identical with naturally occurring ruby,is thus not a mineral.

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