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Geology & Mining Techniques

History Of Ruby Mining Techniques

Although rubies can be found in many locations, not all areas can be successfully mined.  If deposits cannot be mined easily; if they are located in remote, inhospitable, or politically volatile regions; if labor is too costly; or if regulation presents obstacles; there can be no viable mining industry.  

rush site ruby mining
A rush site with extensive efforts for ruby mining.

Market forces also come into play because stones do not fetch optimal prices when the market is saturated. Theft, smuggling, and corruption take a toll on individual mining operations; but they can also affect market supplies or undermine consumer demand, as is the case with conflict diamonds.  

When rubies are found in solid host rock, they are considered to be primary deposits ; miners will build tunnels and drifts directly into the rock to retrieve the stones.  However, most rubies are found far from their original source in secondary deposits .

rock erosion
The visible process of erosion has worn away at this cliff.

Rocks exposed on the earth’s surface are relentlessly scoured by wind and water.  The process of erosion eventually deposits fragments of rock into streams where they are further fractured and release any gems they contain.  In places where the current naturally slows, the heavy gemstones settle into the river or streambed. With time, rubies become concentrated and the site becomes a type of secondary deposit called an alluvial deposit .

Alluvial gem deposits are the most lucrative sources of rubies because they contain concentrated amounts of gemstones and they are relatively easy to extract.  The principles of alluvial mining are the same all over the world; only the level of mechanization differs from place to place.

soil profile in river alluvium
A soil profile in river alluvium where crystal rough is often deposited.

Primitive alluvial mining can require nothing more than a pan, sieve, or basket.  In a technique that has been used for centuries, the banks and muddy bottoms of creeks and rivers are searched for stones. Gem-bearing gravel is dredged out with long-handled baskets. The concentrated gravel is then washed and sorted.     

Gem gravel is also mined by excavating crude pits on land with shovels, picks, and baskets.  When gem deposits are found near the surface, open pits are used, but if the gem gravel is located in deeper strata, the pits, fortified with timber and bamboo, may extend as far as 60 feet into the earth.  

In areas where the water table is shallow, pumping or bailing of the pit may be required. As the pit becomes deeper and reaches the gravel layer, material is brought to the surface and washed in a sieve-like basket.  What remains in the basket is later searched for gems.

Mechanized open pit mining allows larger quantities of earth to be processed with great efficiency.  In many cases, bulldozers scrape the top layers of earth to expose gem-bearing gravel. In some areas the gravel is washed by hand, and in others, where water and technology are readily available, it is washed with high-pressure water cannons and pumped into sluices.  

open pit mining example
A prime example of a large open pit mining operation.

Pulsing jigs, devices that use water and an upward pulsing action to separate materials, may be used to concentrate the corundum at the end of the sluice. The sluices are checked periodically and gem-bearing material is removed and sorted. The use of bulldozers and water cannons can be devastating to the environment.  For this reason, and the fact that primitive mining techniques offer employment for many more people, many countries restrict their use.

Geology Of Rubies

Corundum (Al2O3) is made of aluminum and oxygen.  The three most abundant elements in the Earth’s crust are oxygen (46.6 percent), silicon (27.7 percent) and aluminum (8.1 percent).  But while the elemental components of corundum are abundant, it can only be formed under specific conditions. Silicon readily bonds with both aluminum and oxygen leaving little opportunity for oxides to form.  In fact, corundum is known to grow only under special circumstances where silica is lacking.

ruby natural crystal rough kenya
A stunning sample of natural ruby crystal rough.

Igneous rocks are formed from cooled magma, and certain igneous rocks , called alkali-basalts, are known to be associated with rubies.  The gem-quality rubies found in Thailand, for example, derive from alkali-basalts. Magma deep within the Earth cools slowly, and under special circumstances this can result in the growth of ruby crystals.  

While the majority of the magma is still liquid, it may move upward toward the surface of the Earth carrying ruby crystals with it. The uplifted magma eventually cools and solidifies in fractures near the surface or it erupts as volcanic lava onto the surface of the Earth.

Rubies can also form within metamorphic rock .  Metamorphism is the transformation of one rock type into another via heat, pressure, and chemically active fluids.  When large geographical areas are affected, such as when two continental plates collide, the metamorphic changes are called regional metamorphism.  In other situations, rock may be invaded by magma, causing changes to take place at or near the point of contact.  

metamorphic rock
A large expanse of metamorphic rock.

Contact metamorphism is the term applied to this process in which rock is re-crystallized into another material. If conditions are right, and additional elements are introduced into the rock by the magma, ruby crystals can form.  Exceptional rubies formed via metamorphic processes include those found in the marble-hosted ruby deposits of Myanmar, Kenya, and Tanzania.

Rubies formed with alkali-basalts typically have higher levels of iron as a trace element than marble-hosted rubies created under metamorphic conditions.  The presence of iron inhibits fluorescence, so even if a basalt-hosted ruby has excellent color, it will not glow with the same fire as a marble-hosted gem.  

The trace elements in rubies created via metasomatic processes will differ with the nature of the rock that surrounds them. Some might have low iron content like marble-hosted rubies, and others may have higher iron levels causing them to resemble rubies associated with alkali-basalts.

As we begin our discussion of specific regions, possibly the most famous is our first stop with Ruby Mines in Sri Lanka.

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