| Gold is a noble metal and does not react with oxygen under ambient conditions. Therefore gold does not tarnish, leading to its use in jewelry and coinage. The same resistance to oxidation and other harsh chemical environments leads to its use in industrial applications such as labware and electrical contacts. Although it does not typically form oxides, it will form compounds with other metals and is rarely found in its pure form. It is usually associated with silver and often with mercury. When the silver content is a high, the metal is called electrum, a naturally occurring gold-silver alloy. Gold is also found in tellurides and ore containing quartz where it is either openly visible or enclosed in particles of sulfide minerals such as chalcopyrite, pyrrhotite, pyrite, and arsenopyrite. Gold in its pure form is a very soft metal, so it is often alloyed with other metals for increased strength and hardness. When gold is alloyed, its ductility is diminished, but its malleability remains constant, except when large amounts of copper are added. Red or pink gold (also called rose gold) is gold that contains large percentages of copper and a small amount of silver. Green gold is usually gold alloyed with additional silver or with silver plus a small percentage of copper. White gold is usually 75% gold and 25% nickel (or nickel, zinc, and sometimes copper). Formerly white gold was made with silver and/or palladium, but white gold made with nickel and zinc is harder, cheaper, and has better melting characteristics. |
