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Gemstone Crystal System Density Hardness Refractive index Treatments
Corundum trigonal 4.00 9.0 1.762-1.770 heat, fracture filling;
bulk diffusion with beryllium, coating, irradiation


Examples of corundum

Every color of the rainbow

Clarity: Transparent to Opaque; Type II
Optic Character:
DR Uniaxial Negative
Cause of Color:
Purple: Fe, Ti Cr in varying combinations
Pink: Cr, possibly Ti
Blue: Ti, Fe
Green:Fe or Fe and Ti
Yellow: Fe
Orange: Fe or Color Centers
Orangey Pink: Cr
Color Change: Cr, V, Fe, Ti
Violet and Colr Change::Inert to Moderate to Strong Red
Pink: Strong Orange-res
Blue: Inert to Strong Red; dependent on Fe content
Yellow: Inert to moderate orange, red or yellow
Orange: usually inert
Orangey Pink: May be strong orange-red
Fracture : Concoidal

Luster : Vitreous
Cleavage :
Durability: Excellent

Characteristics for Identification:
Silk (rutile or boehmite), zircon crystals (sometimes with halos), hexagonal growth lines. fingerprint inclusions, hexagonal growth lines, color zoning

Madagascar, Burma, East Africa, United States, Australia


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(aluminum oxide)

Corundum is an aluminum oxide that occurs in every color of the rainbow. When it is red it is termed a ruby. When it occurs in any other color it is termed a sapphire. There are two primary ways that corundum is formed. One is the metamorphosis of limestone and the other is an igneous occurrence in rocks lacking in silica. Since corundum is so hard it is very resistant to weathering. Therefore, it accumulates in placer gravels. Placer deposits in Sri Lanka at Ratnapura have been mined since before the time of Buddha. The major sources today for rubies are Burma and Madagascar. The major source for sapphire is Madagascar. Other locals include Pakistan, Afghanistan, India, East Africa and Yogo Gulch in Montana.

Corundum can be confused with:
Spinel (Natural and Synthetic)

Corundum crystal

Corundum Crystal
Size: thumbnail, 2.1 x 1 x 1 cm

Photo courtesy of Rob Lavinsky