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.
As Sapphire, and especially as ruby, corundum exhibits a ‘Pleochroic’ effect where the
gemstone changes color in relation to a light source.
In rubies case this 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 and intersecting at differing angles
with each other causing the reflected light to form
a star like formation across the gems face.
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 Sapphire and ruby, 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.
Some techniques used today are:
* Heating corundum in oxygen at up to 2050 degrees Celsius is implemented
in order to lighten the coloration of dark Blue Sapphire, and to eradicate the blue component of purplish Ruby.
* Heating corundum at up to 2050 degrees Celsius in a vacuum
void of oxygen or in the presence of carbon monoxide is executed
to intensify the blue color.
* Dissolving an excess of ‘Silk’ out of the crystal
structure is done by heating corundum to 1,600 degrees Celsius and followed
by rapid cooling.
* Intensifying the asterism effect in corundum is performed 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
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.
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 Sapphire and ruby sources, originating from some of the world’s
most exotic lands steeped in legends that put the
god’s to shame.
These are the principal corundum producing countries
and their mining locales regarding quality and desirability
* 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.