Tarnished silver (Mexico) 1
Tarnished silver from Mexico. (public display, Carnegie Mus. of Natural History, Pittsburgh, Pennsylvania, USA)
A mineral is a naturally-occurring, solid, inorganic, crystalline substance 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.
Silver is part of the gold-group of metallic elements. Silver is a precious metal, but is far less valuable than gold or platinum. Silver usually occurs as a silver sulfide mineral, but it also occurs in nature in its native state, often in the form of twisted wires. Silver is moderately soft and has a silvery-white color on fresh surfaces that tarnishes to darker colors. Elemental silver in nature is often found alloyed with other metals. Naturally alloyed gold-silver is called electrum.