Gold

Among the Egyptians and other cultures in the cradle of western civilization gold was used at least as far back as 4000 B.C. It is used today as the basis of the world's monetary system, for ornamentation and for jewelry. For centuries then it has had a high intrinsic value.

Gold is a very soft metal, and it is usually more useful when alloyed with copper, silver or nickel, which makes it hard enough for normal uses. This is especially so in coinage, ornaments, and other objects that suffer rough use. When absolutely pure, gold is labeled for historical rather than scientific purposes as 24 karat gold. Most jewelry is alloyed with copper to give it strength and hardness. Quality jewelry is of fourteen parts of gold to ten parts of copper (14 karat) or of eighteen parts of gold to six parts of copper (18 karat). Inexpensive jewelry that turns black or green after exposure to weather or human skin contains too much copper and too little gold. Gold is also graded in fineness based on how many parts out of one thousand are gold. Usually used for coinage, an alloy of 99 percent gold and 1 percent copper would be .990 fine gold.

The color of gold depends upon the amount and type of impurities it contains.  Native gold is typically golden yellow, but in order to vary its color and increase its hardness for use in jewelry, gold may be alloyed with other metals.  Silver, platinum, nickel, or zinc may be added to give a pale or white gold.  Copper is added for rose or pink gold, and iron is added for a tinge of blue.

Gold occurs in igneous rocks and in associated quartz veins, often in small quantities invisible to the naked eye.  It is also concentrated in secondary deposits in the form of nuggets or grains in river sands and gravels.  Gold may still be extracted from placer deposits 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, Alaska, California, Canada, Russia, South America, and Australia.