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.
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 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.
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.
*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
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.
*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.