Of the 90 chemical elements that occur naturally, less than 20 are found in a pure, or unreacted, form. These metals, semi-metals and non-metals occur because they are sufficiently stable chemically to resist reaction with other chemical agents, at least partially. Under some circumstances they may therefore resist forming compounds and can thus be found in their native state. Only the metals of the Gold and Platinum groups of the Periodic Table, which are the most unreactive, can be found in any significant amounts.

Metals are easily recognised by their metallic luster and malleability (which means that they can be deformed by hitting without disintegrating). Unlike metals, native non-metals are brittle and are poor conductors of electricity. They are usually found in the Carbon and Sulphur groups of the Periodic Table. The last group falls between the metals and non metals in the Periodic Table and exhibit a mixture of both properties. These semi-metals typically inlude Antimony, Bismuth and Arsenic, although some chemists also include Selenium and Tellurium in this group.