Corundum


All gemstones are the crystalline form of certain minerals. Ruby and sapphire are both crystalline forms of the mineral called ‘Corundum’. The present day word ‘Corundum’ is derived from the Sri-Lankan Tamil language and its word ‘Kurundam’, meaning ‘Ruby Sapphire’ this in turn has its roots in the ancient Indian Sanskrit language and its word ‘Kuruvindam’.

Corundum’s Properties

Corundum itself consists of crystallized aluminium oxide (Al2O3). In its pure form corundum has a colorless opaque, translucent and sometimes transparent aspect. When transparent, gem quality corundum contains trace elements of certain metallic substances it becomes colored, and depending on these pigments is categorized as either Ruby or sapphire.

The sapphire, displaying all the colors of the rainbow from pink to midnight blue, gets its myriad of colors from iron and titanium oxide. The Ruby, ranging uniquely in red tones from fiery vermilion to violet red, gets its coloring from chromium oxide. Transparent gem quality corundum suitable for jewelry is relatively scarce, and because the red Ruby, in relation to sapphire's blue and fancy colors, occupies a minute corner of corundum’s color spectrum it commands a much higher price.

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Corundum is the second hardest mineral known to man after diamond, scoring a 9 out of a possible 10 on the Moh’s hardness scale. Due to its hardness, inferior opaque quality corundum that cannot be used as gem material is used in industry as abrasive sandpaper, and in the beauty industry as the chief ingredient of emery paper.

Corundum’s hardness is partly due to the strong and short oxygen-aluminum bonds, which pull the oxygen and aluminum atoms close together, this not only makes corundum’s rhombohedral crystal hard but also dense.


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Corundum possesses an ‘Adamantine’ to ‘Vitreous luster’ meaning that regardless of cut or transparency it will exhibit a bright steely luster. Good quality transparent corundum possessing few inclusions will be faceted, while translucent or flawed corundum will be fashioned into cabochons or beads.

Corundum’s Special Effects

As sapphire, and especially as Ruby, corundum exhibits a ‘Pleochroic’ effect where the gemstone changes color in relation to a light source. In Ruby's case this is especially noticeable under the ultraviolet rays of direct sunlight that give the gem an added fluorescence. Pleochroism is defined as a body color effect, where by ‘Different body colors are seen in different directions of the gems body.’

Sometimes, gem quality corundum exhibits a star effect known as ‘Asterism’, this effect is enhanced when a gemstone is cut and polished ‘En Cabochon’: a dome cut with a flat bottom. The ‘Asterism’ effect is caused by two or more sets of tiny parallel inclusions, known as ‘Rutile’ or ‘Silk’, running the internal length of the gem. These inclusions intersect with each other at differing angles, causing the reflected light to form a star like formation across the gem's face.


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Some corundum, in the case of sapphire, will even change color when exposed to different light sources. This is due to the selective absorption of certain light waves by the gemstone, which when absorbing light waves say from an artificial light source, will produce a different color sensation than when it absorbs light from a natural light source. A classic example of this effect is a gemstone from the chrysoberyl mineral: Alexandrite, which changes color dramatically from green to red.

Corundum Treatments

Heat treatments, wrongly construed as being artificial, mirror the exact same processes that occur in nature. In today’s gem and jewelry industry heat treatments that have proven to be stable are widely practiced and accepted.

Ever since demand grew for Ruby and sapphire, corundum has been artificially treated to improve its color and clarity. In fact it is safe to say that 99% of all corundum gemstones processed and cut today have been subjected to some form of heat treatment.


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*Heating corundum in oxygen at up to 2050 degrees Celsius, is implemented in order to lighten the coloration of dark blue sapphires, and to eradicate the blue component of purplish Rubies.

*Heating corundum at up to 2050 degrees Celsius, in a vacuum void of oxygen or in the presence of carbon monoxide is done to intensify the blue color.

*Dissolving an excess of ‘Silk’ out of the crystal structure is executed by heating corundum to 1,600 degrees Celsius followed by rapid cooling.

*Intensifying the asterism effect in corundum by crystallizing the ‘Silk’, consisting of the mineral titanium, within the gem by heating it to 1,300 degrees Celsius in an oxidized environment and allowing it to cool slowly.


Please note: If you do have the luck of coming across genuine unheated corundum, you will be paying premium prices for a gemstone that doesn't come close to the color or clarity of a similarly sized enhanced variety.

Corundum Origins

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To most devotees of the gem world part of the magic of owning a gemstone lies in marveling at its legends, history and origin. And there are few gemstones, if any, which can compare to corundum’s Ruby and sapphire sources. These are the principal corundum producing countries and their mining locales regarding quality and desirability of origin:

*Kashmir (Northern India): Sapphire.
*Burma (Myanmar) - The Mogok Tract and Mong Hsu: Ruby and sapphire.
*Ceylon (Sri-Lanka) - Ratnapura and Elahera: Blue and fancy sapphire and Ruby.
*Vietnam - Luc Yen and Quy Chau: Ruby and sapphire.
*Madagascar - Andilamena, Vatomandry and Ilakaka: Ruby and Sapphire.
*Tanzania - Umba Valley, Songea: Ruby and sapphire.
*Afghanistan - Jagdalek: Ruby.
*Pakistan – Hunza Valley: Ruby.
*Australia - Inverell in New South Wales: Sapphire.
*Cambodia - Pailin: Sapphire and Ruby.
*Thailand - Kanchanaburi and Chantaburi: Sapphire.

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