The most common corundums are sapphires, rubies, and garnets. See the article below explaining what is corundum.
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What are Corundums?By Margaret Burgon Klemp
In the Webster's New Collegiate Dictionary it says that corundum is "A very hard mineral that consists of aluminum oxide occurring as massive and as variously colored crystals which include the ruby and sapphire that can be synthesized, and that is used as an abrasive."
The Encyclopedia Britannica, 15th Edition, Vol. 3 has this to report about the corundum family of gems. "It is an aluminum oxide mineral that next to the diamond is the hardest known substance; its finer varieties are the gemstones sapphire and ruby. The corundum is widespread, and is found as barrel shaped crystals, large blocks or rounded grains in igneous, sedimentary and metamorphic rocks."
Where Corundums are Found
Corundum is found on all continents particularly in Burma (modern day Myanmar), South Africa, Canada, The Ural Mountains in Western Russia, and the United States.
The most important discoveries, however, are found in Sri Lanka. The attribute that makes them different is that all the stones were produced by a single natural process. There were a number of different ways that these gems originally occurred; mainly pegmatitic, metamorphic and metasomatic. Then as a result of ancient weather conditions the stones were placed in sedimentary deposits, and were grouped as gemiferous deposits. Gemiferous is a term that is used simultaneously with 'carrying gems,' or known as 'placers'. This whole process leaves the stones in rubble, and heaps in valleys and sometimes in riverbeds where two bodies of water come together. The gems are washed down from the mountains into the streams. These corundum beds are estimated to be over a million years old.
Pit and river mining are the most used techniques in Sri Lanka, but the industry is impeded because the country resists modernization. The opposite condition is found in Thailand where all of the equipment and techniques modern and up-to-date. In Thailand the mining sites are likely to be in the jungle. Today it is believed that the mines in Thailand are close to depletion.
Colors of Corundums
Corundums come in a whole spectrum of colors including red which are most likely rubies or garnets, and sapphires which come in a whole bevy of true and shaded blues. Other colors include yellow, orange, pink, green, violet and brown. They tend to radiate brilliantly and genuinely, are rare, and demonstrate exquisite beauty. The word corundum most likely has its roots in the Sanskrit and Hindu languages. In Sanskrit it is known as "kuruwinda," and in the Hindu language as "kurund". They all share common traits. They are made up of crystallized alumina which is mixture of the light metal aluminum together with oxygen. Combined as a hard, durable mineral they transform into a triagonal stone with a six fold surface. There are times that they become barrel-shaped objects, and the ruby has a hexagonal prism. Corundums are not as hard as diamonds, but they are still very substantial and that makes them easier to mine. Their brilliant shine is caused by an extreme sensitivity to light. The light refraction level is in the neighborhood of 1.765. The corundum interacts with other natural phenomena such as light, and that is what causes asterisms in both sapphires and rubies. The light causes a mirage of a six-sided star when it interacts with the cross-sections of the inclusions in the stone. It gives the impression that it is hovering over the surface, and whenever the gem is moved the image seems to glide over the surface.
Following are some terms to recall when studying corundums as well as some other types of gemstones.
Placers - There are two types of placers; rich placers which are groups of stones that are found in gravel, sand, river sediment, and valley floors, and loose weather resistant placers that are found in sand, clay, gravel, and rubble.
Star gem - these are stones that have an asterism that creates the appearance of star shapes caused by light reflecting off the inclusions.
Inclusions - when the gemstone crystallizes alien elements such as minerals, liquids and cracks become part of the natural process, and this produces a flaw which actually is used to authenticate the piece.
Rutile needles - rutile needles are crystallized reddish brown to black elements that are composed of titanium oxide, and sometimes contribute to the formation of inclusions and asterisms.
Sedimentary rock - layered rock caused by natural weathering processes such as a combination of erosion, wind, ice, and water. It keeps reforming and reinventing itself until it becomes a placer in the earth.s crust.
Metamorphic - this means that an element is in transformation. Stones undergo changes due to their contact with magma and natural conditions such a high pressures and temperatures.
Parent rock - this is rock where a gemstone is found to have actually formed at the same time as the rock.
Pegmatitic - means that ingredients that combined to form the stone are fastened together.
Metasomatic - this is rock where chemical composition and texture changes occurred.
Bibliography: (not mentioned above)
Gems: A Lively Guide for the Casual Collector, Daniel J. Dennis Jr. , 1999 Harry N. Abrams, Inc., New York, New York
Gemstones: Symbols of Beauty and Power, Eduard Gubelin and Franz-Xaver Erni, Geoscience Press, Tucson, Arizona