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Corundum: Facts & Information

All about the Corundum varieties: Sapphire, Padparadscha and Ruby

Corundum is the second hardest substance known to man (second only to diamond).

Corundum occurs naturally in all colors. The "gem" varieties of corundum are known as sapphire, ruby and padparadscha. These natural corundum varieties are based on color.

Links To Corundum Facts and Information Topics On This Page:
Each Corundum Variety and the Corundum Products in our VIRTUAL GALLERY
Corundum Physical Properties
Corundum Background Information
Corundum Occurance and Diagnostic Features
Corundum History and Uses
Corundum Metaphysical Properties

Genuine Sapphire Jewelry (with Natural Sapphire Gemstones)

Return to the Index of Corundum Facts and Information

PHYSICAL PROPERTIES of CORUNDUM

• Corundum Chemical composition:Al₂O₃, color is due to trace impurities of Cr, Fe and Ti. LI>Class: oxides
• Crystal system: hexagonal-rhombohedral, bar3 2/m
• Corundum Crystal habit: sapphire crystals are commonly prismatic while ruby crystals are more often tabular {0001}, terminated by basal pinacoids, pyramidal and rhombohedral faces may be partly or entirely absent. Sapphires often occur as tapering hexagonal pyramids with a barrel-shaped form. Basal planes often have parallel striations meeting at 120 degrees forming successive equilateral triangles. Prismatic faces often have deep horizontal striations. Also massive and coarse to fine granular.
• Twinning: commonly with multiple lamellar twinning on rhombohedral planes, less commonly with simple contact twins.
• Corundum Specific gravity: 3.98-4.10
• Index of refraction: 1.757-1.778
• Corundum Birefringence: weak, 0.007-0.010
• Corundum Pleochroism: pronounced. Blue sapphire: violet blue/blue-green, green sapphire: strong green/yellow-green, orange sapphire: yellow-brown to orange/colorless, purple sapphire: violet/orange, yellow sapphire: yellow/pale yellow and ruby: purple-red/orange-red
• Corundum Hardness: 9
• Corundum Color: Ruby: pinkish red to red. Sapphire (all other colors): blue, pink, green, yellow, violet, purple, orange, brown, white, gray, black and colorless. Some sapphire crystals are color zoned concentrically and/or along the C axis.
• Luster: adamantine to vitreous, can occur with minute rutile inclusions oriented at an angle of 120 degrees from each other according to the hexagonal symmetry of the corundum. When this material is fashioned into a cabochon gem with the c axis of the corundum perpendicular to the base of the stone, reflected light from the rutile inclusions can concentrate on the surface of the gem forming white lines that intersect every 60 degrees forming a six ray star. This material is known as star ruby if it is pinkish-red to red, or as star sapphire with a color prefix for all other colors except blue which is simply called star sapphire.
• Corundum Transparency: transparent to translucent
• Corundum Cleavage: none - however, well developed parting on basal & rhombohedral planes is common.
• Corundum Fracture: conchoidal, brittle
• Streak: white

Each Corundum Variety (Sapphires, Padparadsha & Ruby) and All Corundum Products In Our Store
(Natural Corundums)

CORUNDUM BACKGROUND INFORMATION

Pure Corundum (Al₂O₃) is rare in nature and is perfectly colorless. Small amounts of metalic elements can substitute for aluminum in the corundum crystal structure, giving rise to many color variations. Gem corundum of any color is refered to as sapphire with a preceeding color designation if the color is any other than blue. Blue corundum is simply called sapphire. There are two other color varieties of corundum that have specific names, the pinkish-red to red ruby and the orangish-pink padparadscha.

The famous pinkish-red to red ruby owes its color to chromium (Cr³⁺) impurities, and the color of the rare orangish-pink padparadscha sapphire is from both iron and chromium impurities. The color of the well known blue sapphire (or simply sapphire) is due to Fe²⁺ and Ti⁴⁺ impurities. Iron in the Fe³⁺ state causes the color in some yellow sapphires, while other yellow sapphires owe their color to irradiation induced 'color centers'. Other color influencing impurities in sapphires are vanadium, cobalt and nickel.

Some sapphires owe their color to 'color centers' which are not fully understood, but it can be said that they are created by radiation and destroyed (reversed) by heating or in some cases by exposure to light. Some color centers in sapphire are stable until heated hundreds of degrees while others are very unstable, fading within a few days merely by exposure to light.

The color and in some cases the clarity in gem corundum can be modified by a number of processes that are in common use today. Most of these processes involve heating, often in the presence of specific chemical ingredients, in order to intensify a natural color or sometimes to introduce an entirely different color altogether. Many light gray to pale blue gem corundums with a 'silky' look due to microscopic titanium oxide inclusions are heated to near their melting point, resulting in the dissolution of the silk-causing inclusions producing a more transparent gem. This improvement in clarity is often accompanied by a much stronger deep blue color as well.

Gem corundum that exhibits natural color and clarity commands a significant price premium over comparable artificially enhanced corundum of natural origin. Determining whether a natural gem corundum has been artificially enhanced can be very difficult and is quite often impossible with the naked eye alone. Special techniques and equipment are usually neccessary, and even then some doubts can remain.

Some corundum occurs with minute rutile inclusions oriented at an angle of 120 degrees from each other according to the hexagonal symmetry of the corundum. If the concentration of rutile fibers is sufficiently dense, then when this material is fashioned into a cabochon gem with the c axis of the corundum perpendicular to the base of the stone, reflected light from the rutile inclusions can concentrate on the surface of the gem forming white lines that intersect every 60 degrees forming a six ray star. This material is known as star ruby if it is pinkish-red to red, or as star sapphire with a color prefix for all other colors other than blue. Blue star corundum is simply called star sapphire. Most synthetically produced star corundum has very perfect stars with fine straight rays. Natural star corundum almost always has eye visible imperfections even if very faint, although occaisionally a natural star corundum does occur with an exceptionally fine quality near-perfect star.

CORUNDUM OCCURRENCE AND DIAGNOSTIC FEATURES

Corundum occurs in a variety of geological settings, characteristically in aluminum-rich and silica-poor rocks. It often occurs as an accessory mineral in metamorphic rocks derived from aluminous or carbonate sediments such as crystalline limestones and marbles, mica schists and gneisses. It is also found in contact zones between igneous rocks and limestones.

Corundum also appears in silica-poor light colored igneous rocks such as desilicated pegmatites, syenites and nepheline syenites.

Due to its high hardness and chemical inertness, corundum is sometimes found in commercially viable quantities in alluvial deposits.

Corundum is characterized by its high hardness, specific gravity, color, its luster and habit.

CORUNDUM HISTORY INFORMATION AND USES

The name corundum was derived either from the Sanskrit kurivinda or from the Indian name for corundum kauruntaka. Ruby is from the Latin word ruber meaning red, and sapphire is from the Latin word for blue sapphirus, which is also thought to have been used in ancient times in refering to lapis lazuli.

The corundum gem varieties ruby and sapphire are among the oldest gems known to man, dating back many thousands of years. Ruby and sapphire were both held in very high regard, especially the ruby. The ancient Sanskrit names for ruby translate as king of precious stones or leader of precious stones. Both ruby and sapphire were in the 'Breastplate of Judgement' of Aaron, described in the bible (Exodus: xxviii, 15-30).

Throughout history both ruby and sapphire have been attributed with having a great variety of beneficial properties. Ruby was said to preserve health and give invulnerability from wounds, and to guarantee that one's status and posessions would never be taken. The wearing of a ruby was said to signify manhood, nobility and valor in a man, and pride & passion in a woman. Sapphire was said to bring divine favor, to make peace between enemies and to protect against envy, fraud, poison and sorcery. The wearing of sapphire jewelry was said to signify generosity, good manners, wisdom and noble thoughts. Star sapphire was widely believed to serve as protecion against witchcraft or sorcery of any kind.

Corundum is used as a gemstone, occuring in all colors. Due to natural corundum's popularity and high value, synthetically produced gem corundum has been used as an inexpensive substitute for many years. In ancient times, natural spinel gemstones were often confused with natural sapphires and ruby.
Corundum is also used as an abrasive. Corundum that has been synthetically manufactured from bauxite is most often used for this purpose. The abrasive material emery is a mixture of corundum and magnetite.

Ruby is the birthstone for the month of July.
Sapphire is the birthstone for the month of September.

Sapphire is the symbolic gemstone for the 23th wedding anniversary.
Star sapphire is the symbolic gemstone for the 26th wedding anniversary.
Ruby is the symbolic gemstone for the 40th wedding anniversary.
Star ruby is the symbolic gemstone for the 52nd wedding anniversary.
Yellow sapphire gemstones are used with vedic astrology and jyotish.
The astrological sign of corundum is Sagittarius.
The astrological signs of ruby are Leo, Scorpio, Cancer and Sagittarius.
The astrological signs of sapphire are Virgo, Libra and Sagittarius.

CORUNDUM METAPHYSICAL PROPERTIES

Corundum is said to enhance one's creativity, self-confidence, awareness, intuition and ambition. It is also said to help release negative emotions in constructive ways while giving give inner peace.

Metaphysical properties for some of the corundum varieties are given below:
Black sapphire: is said to protect one from physical harm and enhance one's self-confidence.
Green sapphire: is said to strengthen loyalty and honesty.
Pink sapphire: is said to inspire love, and to enhance one's creativity and self-reliance.
Purple sapphire: is said to aid in meditation, and to enhance one's spirituality.
Ruby: is said to inspire love, and to enhance creativity, wisdom and spirituality. It is also said to give confidence, self-esteem and courage, and to stimulate leadership qualities. Ruby is also said to have been helpfull in treating heart and circulatory disease. See our genuine ruby necklaces and ruby earrings.
Sapphire (blue color): is said to eliminate confusion, and to dissipate negative emotions such as fear, hate and anger, replacing them with peace, joy and love. It is said to enhance one's ability to communicate with others. Sapphire is also said to have been helpfull in treating disorders of the eyes and skin, and in stopping infections.
Star sapphire: said to aid in meditation and to give mental clarity and wisdom. It is also said to bring good fortune and to give one a more pleasant disposition.
Yellow sapphire: is said to bring wisdom, wealth and prosperity.
White sapphire (colorless): is said to stimulate one's intellect and to help focus one's concentration. It is also said to inspire fairness and morality. See our selection of natural white sapphire earrings!

For more in-depth metaphysical information on the different corundum varieties, see our Metaphysical Books section.

This is the end of our Corundum Information page.