Composition - Gold ; Hardness - 2 1/2
The colour of gold depends upon the amount and type of impurities it contains. Native gold is typically golden yellow, but in order to vary its colour and increase its hardness for use in jewellery, gold may be alloyed to other metals. Silver, platinum, nickel or zinc may be added to give a pale or white gold. Copper is added for red or pink gold; iron for a tinge of blue. Gold purity is defined by the proportion of pure gold metal present, and this is expressed as its carat (ct) value. The purity of gold used in jewellery varies from 9 carat (37 ½ per cent or more pure gold) through 14, 18 and 22 carat, to 24 carat, which is pure gold. In many countries, gold is “hallmarked” to indicate its degree of purity.
OCCURRENCE
Gold is found in igneous rocks and in associated quartz veins, often in small quantities invisible to the naked eye. It is also concentrated in secondary “placer” deposits as nuggets or grains in river sands and gravels. Gold may still be extracted from placer deposit by the traditional panning method, but modern commercial mining involves large earth-moving machinery and concentrated acids for processing the ore. The main gold-bearing rocks occur in Africa, California and Alaska (USA), Canada, the former USSR, South America and Australia.
REMARK
Gold has been used for coins, decoration, and jewellery for thousands of years. It is attractive, easily worked, and wears well.
