Platinum

Platinum has been used for thousands of years, but it was not recognized as a chemical element until 1735.  Of the three precious metals, gold, silver and platinum, it is the rarest and the most valuable.  Chemically inert and resistant to corrosion, platinum does not tarnish when exposed to the atmosphere, unlike silver.  It is silvery gray, gray white or white in color, opaque and has a metallic luster.  It is slightly more dense than pure gold and about twice as dense as silver.  Early jewelers had difficulty achieving the 3223° F (1773° C) needed to melt platinum, and it was not until the 1920s that the technology was developed sufficiently to work this precious metal.

Platinum occurs in igneous rocks, usually as ores in which the grains of platinum are often too minute to be seen with the naked eye.  It may also occur in deposits in river sands and gravels and in glacial deposits, usually as grains, more rarely as nuggets.  The main occurrences of platinum have been in South Africa, Canada, Alaska, Russia, Australia, Colombia, and Peru.