If you think about finding gemstones, it's seems like it's similar to searching for oil -- you identify the conditions under which the material forms, then you go prospecting for likely locations. Once you find one, you start digging.
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Some gemstones are found that way. These kinds of deposits are called primary deposits in mineralogy. In primary deposits the gemstones still have their original relationship with their host rock. The crystals are usually well-preserved, but extracting the gem material usually involves moving many tons of nongem-bearing rock.
However, gemstones are not always found where they form in the Earth. In many cases geological or meteorological processes will transport the material to a new location. These are known as secondary deposits, and are very important in the mining business.
Discovering a secondary deposit can make mining much easier because the gem crystals have already been separated from the host rock. Secondary deposits are classified according to the way they have been transported or according to the place of deposit. Mineralogists identify deposits as fluvial (river), marine (in the sea), littoral (coastal) or aeolian (by the wind).
Gemstone deposits which were deposited by rivers are called placers or alluvial deposits. Rivers can transport gem-bearing rocks many hundreds of miles. When the transporting energy of the current diminishes, the denser gems -- diamond, zircon, garnet, sapphire, chrysoberyl, topaz, peridot and tourmaline -- are deposited before the lighter quartz sand. Thus the gems left behind by the river tend to be concentrated in certain places. This makes mining the deposit much easier and more productive.
Most gemstone deposits were found by accident. Even today systematic prospecting is mainly limited to diamond occurrences. Diamond production is run by international companies who can afford large capital investments, because the price of diamonds is controlled on a worldwide scale. In the colored gemstone world, most mining continues to be artisanal, with small local mining operations using primitive methods.
The easiest way to collect gemstones is to discover stones at the surface in a dry riverbed. Prospecting in rivers can also be productive. In many cases alluvial deposits are found below the surface and a great deal of manual labor is required to extract the rock and soil to examine it for gemstones. Most corundum (ruby and sapphire) are found in alluvial deposits.