Chemical Formula The generalized chemical content of a pure example of a mineral class. For example, CaCO3 the formula for Calcite, but it can vary slightly from this compostion and still retain the internal structure characteristic of Calcite. Cleavage Minerals break along surfaces parallel to the geometric configuration of their atomic structure. For example, Mica has a sheet-like cleavage which means that it breaks along horizontal planes only. Color The most commonly occuring color for a pure example of each mineral. Crystal System The internal structure of all minerals can be classified into a set of six crystal systems. For example, quartz has a hexagonal crystal structure which is clearly reflected in its normal shape. Hardness This number is derived from Mohs scale of hardness which ranges from 1 to 10. 10 being the hardest (eg Diamond). Luster The characteristic surface apperance of a mineral. For example, Pyrite has a metallic luster. Mineral A naturally occuring substance with a characteristic internal structure determined by a regular arrangement of atoms and ions and with a chemical composition and physical properties that are either fixed or varied witihin a definite range. Occurence An estimate of the frequency of pure examples in nature. Specific Gravity The ratio of the weight or mass of a cubic centimeter of a mineral to that of an equal volume of water. Streak The color made by a mineral scratched against a porcelain plate. Transparency An estimate of a mineral's clarity.