Facts
Rhodium is a very rare, silvery-white, hard, corrosion-resistant transition metal. It is one of the rarest and most valuable precious metals.
Rhodium ores are a mixture with other metals such as palladium, silver, platinum, and gold. Few rhodium minerals are known. Principal sources are located in South Africa, the Ural Mountains in Russia, and in North America, especially the copper-nickel sulfide mining area of the Sudbury, Ontario, region.
Because rhodium metal is inert against corrosion and most aggressive chemicals, and because of its rarity, rhodium is usually alloyed with platinum or palladium and applied in high-temperature and corrosion-resistive coatings. The primary use of this element is in automobiles as a catalytic converter, changing harmful unburned hydrocarbons, carbon monoxide, and nitrogen oxide exhaust emissions into less noxious gases. Rhodium detectors are used in nuclear reactors.
Rhodium has not been found to serve any biological function.
No toxic effects of rhodium have been reported from observations of humans. It has been reported that inhalation of excessive amounts of fine rhodium metal powder or dust may cause irritation of the respiratory system, and that eye contact with fine powder or dust may cause irritation (mechanical irritation).
S.C.Gad. Rhodium, 2014, Encyclopedia of Toxicology (Third Edition)