Silver usually occurs in massive form as nuggets or grains, although it may also be found in wiry, dendritic (treelike) aggregates. When newly mined or recently polished, it has a characteristic bright, silver-white colour and metallic luster. However, on exposure to oxygen in the air, a black layer of silver oxide readily forms, tarnishing the surface. Because of this, and the fact that it is too soft to be used in most jewelry in its pure or native form, silver is often alloyed with other metals or given a covering layer of gold or rhodium. Electrum, an alloy of gold and silver in use since the time of the ancient Greeks contains 20-25 percent silver. Sterling Silver contains 92.5 percent or more pure silver (and usually some copper), and Britannia silver has a silver content of 95 percent or more. Both alloys are used as standards to define silver content.
Most silver is a by-product of lead mining and is often associated with copper. The main silver mining areas of the world are South America, The USA, Australia and the former USSR. The greatest single producer of silver is probably Mexico, where silver has been mined for almost 500 years. The finest native silver, which occurs naturally in the shape of twisted wire, is from Kongsberg, Norway.
Sources: Hall C (2002). Gemstones.United States. Smithsonian Handbooks