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Gemspedia >> Gems >> Gemstone Minning

Gemstone Minning

In antiquity, most gemstones were discovered near the surface, generally by accident. While this has somewhat changed in modern times, prospecting for colored gemstones is still a fairly primitive affair, relying more on observation and chance, than the intensive scientific methods employed by the large multinational corporations involved in diamond exploration.


One of the most intriguing aspects of gemstone mining is the diversity of techniques employed in their extraction. These range from low tech tools such as shovels and sieves, to the high tech methods used to extract diamonds from pipes (a volcanic pathway that connects the earth’s deep mantle to the surface). Apart from the introduction of power tools and pumps, most colored gem mining hasn’t changed dramatically in thousands of years and still relies on three key things—perseverance, hand tools and elbow grease.

 

With a radiocarbon age of 43,000 years, the oldest known mine is the Lion Cave in Swaziland. At this site, people mined the iron-containing mineral hematite, which they presumably ground to produce the red pigment, ochre. Sites of a similar age were also found by archaeologists in the Netherlands and Hungary, which may have been worked for flint in weapons and tools.

 

Another early mining operation was the turquoise mine operated by the ancient Egyptians at Wady Maghareh on the Sinai Peninsula. Turquoise was also mined in pre-Colombian America in the Cerillos mining district in New Mexico, where a mass of rock some 200 feet in depth and 300 feet in width was removed with stone tools. The resulting mine dump of unusable rock covers 20 acres.

 

Gemstones are generally obtained by alluvial or host rock mining.

Washing alluvial gem gravels near Ilakaka, Madagascar

Alluvial mining

By far the most common method of mining gemstones, alluvial mining is the extraction of gems from sedimentary deposits, also known as placer or secondary deposits.

They are called secondary deposits because the gems are not found in the rock in which they formed or are hosted, but in deposits caused by the weathering and erosion of primary deposits. It includes the prospecting of riverbeds (i.e., the water flow is dammed so the less dense clay and sand is swept away—the remaining gem gravel is then agitated so the gems can be extracted and sorted) or the mining of gems from sedimentary deposits located beneath the earth’s surface (i.e., the digging of pits, vertical shafts and tunnels to reach gem gravels). Examples of alluvial mining include river and shaft mining in Ratnapura, Sri Lanka and Ilakaka, Madagascar. A variation of alluvial mining is marine mining, which is the mining of sandy coastal strata by dredging (e.g., amber from Kaliningrad, Russia).

 

Typically, a miner will dig using either hand tools (on a small scale mine) or heavy industrial machinery. The earth is then taken to be washed either by hand or with the aid of machinery. This is exactly how it sounds—the loose earth is washed with water to get rid of the debris, leaving gemstones in the “wash.” This wash is then trawled through to find the rough gemstones. It is an incredibly laborious and time consuming process that can from day to day, yield very little. Typically, only a few little gems remain at the end of washing and sorting.

 

Alluvial deposits tend to include more than one gem type and this can be useful, as the presence of one gem type can often indicate the presence of another. Such gems are called tracers, as they allow prospectors to trace down other varieties. Other methods of prospecting include the mapping of ancient riverbeds and streams.

 

Although gem crystals from alluvial deposits tend to be rounded, scratched and cracked due to the weathering processes they have endured, this is actually beneficial as the culling of poorer specimens has already occurred. In fact, the percentage of gem quality crystals found in alluvial deposits is generally higher than those obtained from primary host rock deposits.

 

Host rock mining

Also known as primary deposits, host rock mining is the chipping of gems from the rock in which they formed or are hosted. Crystals from these deposits are extracted from their host rock by hand tools, pneumatic tools and even explosives. Performed for centuries, this mining is typically done by digging underground tunnels. In some cases, gemstones can be harvested directly from underground caves (e.g., the mining of moonstone from limestone caves near the village of Kangayam in Tamil Nadu, southern India).

 

Depending on the hardness of the primary material, host rock mining can either be comparatively easy (e.g., mining of kunzite from pegmatites at Betafo, Madagascar) or extremely difficult (e.g., mining of tanzanite from metamorphic rocks at the Merelani Foothills in northern Tanzania).

Inside a tunnel at the world’s only tanzanite mine at Merelani, Tanzania

While some gems are easily removed by picks or drills, sometimes the host material must be crushed before the gems can be extracted and sorted for quality. At many diamond mines they use grease tables. This is a method of diamond extraction whereby crushed rock is run down a gently sloping table covered in grease. Because diamonds are hydrophobic (water repellent), any diamonds present in the sample will adhere to the grease.

The rarity of gems

Imagine sitting for hours in the sweltering Brazilian bush watching two miners muscle spade after spade of earth out of an ever-deeper hole. Getting up to leave, you take stock of the day’s finds—one small colorless topaz and an equally small aquamarine.

 

Or take a group of artisinal miners in Madagascar. When their production from the past two days’ work is displayed, the tiny pile of stones does not fill even half a palm. At a nearby mechanized mine, despite machinery and a crew of a dozen, cleaning out the jig yields just a few dozen sapphires, none of which will cut a stone above two carats.

A lot of work for a little beauty—two days’ yield from a sapphire mine near Ilakaka, Madagascar

These are events the author (RWH) has personally witnessed, humbling experiences that drive home the true rarity of fine colored stones (not to mention the difficulty often encountered in bringing them to market). Perhaps Walter Houston put it best in the classic movie, Treasure of the Sierra Madre:

 

Why is gold worth some twenty bucks an ounce? A thousand men, say, go searchin’ for gold. After six months, one of them’s lucky…. His find represents not only his own labor, but that of 999 others, to boot. That’s 6000 months, 500 years, scramblin’ over a mountain, goin’ hungry and thirsty. An ounce of gold, mister, is worth what it is because of the human labor that went into the findin’ and the gettin’ of it.

 

When you purchase a precious gem, you own one of nature’s rarest creations. So much of nature’s beauty is ephemeral, passing quickly away. Gems are among the most enduring examples of the natural world, and arguably the most beautiful, too.


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