A mineral can be defined as a naturally occurring inorganic solid that possesses an orderly internal structure and a definite chemical composition. Some people, like physicists, might be guilty of picking up a rock and calling it a mineral. The term "rock" is less specific, referring to any solid mass of mineral or mineral-like material. Common rocks are often made up of crystals of several kinds of minerals. There are some substances, like opal, which have the appearance of a mineral but lack any definite internal structure, are sometimes called "mineraloids". Lutgens and Tarbuck give the following list of essential characteristics of a "mineral":
- It must occur naturally.
- It must be inorganic
- It must be a solid
- It must possess an orderly internal structure, that is, its atoms must be arranged in a definite pattern.
- It must have a definite chemical composition that may vary within specified limits."
The most common minerals are the silicates, as one would guess by looking at the abundances of the elements in the Earth's crust, but there is a great variety of minerals. Minerals are classified in many ways, including hardness, optical properties, crystal structure, etc. Shipman, et al. comment that over 2000 minerals have been found in the Earth's crust, but that about 20 of them are common and fewer than 10 account for over 90% of the crust by mass.
Non-silicates constitute less than 10% of the Earth's crust. The most common non-silicates are the carbonates, the oxides, and the sulfides. There are also naturally occuring phosphates and salts. There are a few elements which occur in pure form, including gold, silver, copper, bismuth, arsenic, lead and tellurium. Carbon is found in both graphite and diamond form. Some minerals are valued as gems because of their hardness, color and beauty.
Lutgens & Tarbuck
Shipman, et al.