Electrum from Mexico. (CSM # 88.37, Colorado School of Mines Geology Museum, Golden, Colorado, USA)

A mineral is a naturally-occurring, solid, inorganic, crystalline substrance having a fairly definite chemical composition and having fairly definite physical properties. At its simplest, a mineral is a naturally-occurring solid chemical. Currently, there are over 4900 named and described minerals - about 200 of them are common and about 20 of them are very common. Mineral classification is based on anion chemistry. Major categories of minerals are: elements, sulfides, oxides, halides, carbonates, sulfates, phosphates, and silicates.

Elements are fundamental substances of matter - matter that is composed of the same types of atoms. At present, 118 elements are known (four of them are still unnamed). Of these, 98 occur naturally on Earth (hydrogen to californium). Most of these occur in rocks & minerals, although some occur in very small, trace amounts. Only some elements occur in their native elemental state as minerals.

To find a native element in nature, it must be relatively non-reactive and there must be some concentration process. Metallic, semimetallic (metalloid), and nonmetallic elements are known in their native state as minerals.

Electrum (AuAg) is an alloy (mix) of elemental gold and silver with >20% Ag. Most “gold” mineral specimens have some silver mixed in as well. Electrum has a metallic luster; ranges in color from silvery to golden yellow, depending on the silver content; has a high specific gravity (it's heavy for its size); has no cleavage; and is malleable.