Native Gold

The predominant occurrence of gold is as native metal, often alloyed with up to 15 % silver. Other gold minerals include alloys with tellurium, selenium, bismuth mercury, copper, iron, rhodium, and platinum. There are no common naturally occurring gold oxides, silicates carbonates, sulphates or sulphides. Therefore, gold generally occurs in a mineral form different to most other elements which allows selective gold extraction from other mineral mixtures.

Native gold grains have been known to contain up to 99.8% gold but most vary between 85 and 95% Au content, with silver as the main impurity. Pure gold has a density of 19.3, though native gold typically has a density of 15.0. Hence, if liberated from gangue minerals, it can be readily recovered at particle sizes above 10 µm by gravity concentration; the major method of recovery of gold employed throughout history. Gravity concentration can be selective, as the most common gangue minerals, e.g. quartz and other silicates, have densities in the range 2.7-3.5.

Gold is very soft, ductile and malleable (1 oz of gold can be beaten into an area of 30 m²) with Vickers and Mohs' hardness numbers of 40-95 kg/mm² and 2.5-3.0, respectively. Native gold rarely occurs in its cubic crystal form and the famous rounded masses, known as nuggets, are now only found occasionally. Gold does not display cleavage on breakage. There are many terms to describe the various, and often distinctive forms of native gold, i.e. sponge gold, flakey gold, grain gold, foil gold, moss gold and tree gold.