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Gold is often found in underground veins of quartz and, less frequently, in other minerals such as pyrite, granite and mica slate. The veins can be anything from half an inch to several feet wide. 80% of the gold that’s produced today is mined from such sources (the rest is ‘alluvial’ gold – see below). Knowing where to mine involves ‘chasing’ the gold-bearing veins on the surface, back underground. It’s been estimated that in the past 500 years, about 100, 000 tonnes of gold have been mined, but even this amount would only fill a cube with 17 m sides. The world’s biggest gold producer is South Africa, where gold mines are sunk over 3,000 metres deep into the earth.

There are also large deposits of gold above the ground. So-called ‘alluvial’ gold is found as small yellow grains and flakes, or even small nuggets, on the beds of fast-flowing rivers and streams. Natural erosion, flooding, glacial movement and weathering also play their part in freeing the gold from mountain rocks. As it’s carried downstream, the gold-bearing rock is broken up into increasingly smaller pieces, thereby releasing the gold from the quartz and mineral veins. The released gold is itself broken up into smaller and smaller fragments. The further it’s carried downstream, the smaller the gold particles become. By the time it reaches the sea, any gold is in the form of very small specks or grains.

You’re more likely to find gold deposits on the inside of bends in a river, where the water flows less quickly. As the water slows down, the heavy gold particles fall through the gravel on the riverbed, and work their way down through the soil underneath, eventually settling on the riverbed’s clay bottom. The gold can be separated from the rest of the riverbed soil and clay by panning (panning involves scooping up a sample of the sand, gravel and clay on the riverbed and swirling it with water in a shallow, wok-shaped dish), a process that relies on the extreme heaviness of gold. Any gold, being so much heavier than the rest of the minerals in the pan, will settle to the bottom, while the lighter material is washed out of the pan by the swirling motion. Any gold in your panned sample will stand out clearly as tiny, yellow flecks or small grains, a process that relies on the extreme heaviness of gold. Iron pyrites (iron sulfide, FeS2), or 'fool’s gold', is often mistaken for gold because it’s the same ‘metallic’, bright-yellow colour. To distinguish between fool’s gold and the real thing, smash it with a hammer. Unlike gold, which will just flatten under the hammer, iron pyrites is brittle, and will smash into small pieces. If you heat iron pyrites to a high temperature, it will give off sulfur, which has a distinctive smell. Gold on the other hand, will just melt at 1 063 ºC.

What’s a carat then? Most naturally occurring gold is impure. It usually contains small but significant amounts of other metals such as copper, silver, palladium and mercury. These each give the gold distinctive colours; for example, gold that contains a significant amount of copper will be tinged red, while gold containing silver will be much paler than the distinctive and unmistakable bright-yellow of pure gold.