A study of gemstone mining and retrieval covers every mining method, from the ancient traditional searches in streams and rivers using just a pan or sieve, to the ultra-high technology and research used in diamond mines deep underground. Ultimately, any source of gemstones will only be mined or exploited if the source is viable. It must be possible to mine or retrieve the gems at a profit to make the venture a viable business.
The oldest and most traditional methods are still used in areas where gemstones are near the surface, relatively easy to find and retrieve, and where labour is cheap and affordable. For example, in
This is possiblebecause the gemstones are generally heavier than the surrounding mud, pebbles, or rock fragments. As a pan of water and sediment is 'jiggled', the gemstones settle in the pan as the lighter constituents and the water are washed over the pan's edge. The heavier concentrate may be sieved to separate the larger gems, or spread out on tables or cloths to be hand-sorted and the gems found by eye. These gemstone localities, found associated with the sedimentary rocks of rivers and streams, are called 'alluvial deposits' or placer deposits. They are secondary deposits: they are not found in the rock in which they were formed, but where they have been transported to as a result of weathering and erosion.
Gemstones that survive the journey tend to be those that are sufficiently hard to withstand the conditions without breaking rather than those that are heavily included, that fracture easily, or break along cleavage planes. They are generally harder and heavier than surrounding minerals and in water they tend to sink faster and are therefore not carried as far. The surviving gemstones will generally become concentrated in pockets or areas along the riverbanks or within sediments, as gem gravels.
Alluvial deposits, such as the gem gravels of
Because a number of gemstones that are associated with gem gravels are often found together, the discovery of one type of gemstone from a gem 'association' can be used by exploration teams and prospectors to 'home in on' or 'trace' potential gemstone mining areas. Another technique is to map the courses of ancient river beds or present-day rivers and streams, and follow tracer gems downstream in the hope of finding areas where the gemstones are in sufficiently large enough concentrations to be retrieved.
Where the rock is harder, these methods are insufficient. For example, rubies can form in metamorphic rocks. In some instances, picks and drills may be sufficient to price loose the gemstones from the parent rock (the host), while in others the rock has to be mined, crushed, washed, and sorted to retrieve the gemstones.
Diamonds are mined on a larger scale and with more highly mechanized methods than any other gemstone, because of their value and range of uses. The better quality stones may be fashioned as cut gems, while even the non-gem-quality diamond can be used as an abrasive or for other industrial purposes. The percentage of gem-quality to industrial-quality diamond varies from mine to mine and from continent to continent.
On the other hand, pipe mining of igneous rock (kimberlite or lamproite) produces a greater total yield of diamonds, but typically only 20-25% are of gem quality.
The diamond pipes are mined from the surface, removing the rock and transporting it to processing plants to be crushed and washed before removing the diamonds. As the pipe is dug out, a large pit is formed. Once a pit is about 300 metres (1,000 feet) deep, underground tunnels and shafts are needed to excavate and remove the diamond-bearing rock.