For millennia, gemstones and certain metals have been prized throughout the world. The concept of treasure itself involves these rare and valuable materials. Substances of great value are termed 'precious'. The precious metals include gold, silver and platinum, and several others in the 'platinum group' (iridium, palladium, rhodium). So-called 'precious' stones historically included diamond, ruby, emerald, sapphire, opal and pearl. Other gems were termed 'semiprecious', but these labels have no meaning today and should be abandoned.