Cobalti- is a prefix used to denote cobalt, a chemical element with atomic number 27. Cobalt is a hard, brittle, silver-grey metal that is often used as a component of alloys. As a prefix, cobalti- serves to indicate the presence of cobalt in a chemical compound or a chemical process. Some synonyms for cobalti- include cobaltine, a mineral composed of cobalt, arsenic, and sulfur; cobaltous, a term used to describe compounds containing divalent cobalt; and cobaltic, a term used to describe compounds containing trivalent cobalt. Other related terms include cobalt-blue, a deep blue pigment made from powdered cobalt; and cobaltocene, a redox-active organometallic compound containing cobalt.