The six platinum group metals (PGM's) are platinum, palladium, rhodium, osmium, iridium and ruthenium. They commonly occur together in nature and are among the most scarce of the metallic elements. Economically, the 3 most significant PGM's are platinum, palladium and rhodium.
Platinum is a silvery-white metal when pure, and is malleable and ductile. Platinum's wear and tarnish resistance characteristics are well suited for making fine jewelry. Platinum possesses high resistance to chemical attack, excellent high temperature characteristics and stable electrical properties, all of which are ideal for the production of industrial applications.
Palladium is a soft silver-white metal that resembles platinum and is the second most abundant PGM after platinum. Palladium does not tarnish in air, and is the least dense and lowest melting of the platinum group metals. When annealed, it is soft and ductile. Cold working increases its strength and hardness. At room temperatures the metal has the unusual property of absorbing up to 900 times its own volume of hydrogen. Hydrogen readily diffuses through heated palladium and this provides a means of purifying the gas.
Rhodium is a rare silvery-white hard metal member of the platinum group. Rhodium has a higher melting point and lower density than platinum. It has a high reflectance and is hard and durable. Rhodium is found in platinum ores and is used in alloys with platinum and as a catalyst. It is the most expensive precious metal.