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Hexagonal Corundum Crystal

The Mineral corundum

Corundum is best known for its gem varieties, Ruby and Sapphire. Ruby and Sapphire are scientifically the same mineral, but just differ in color. Ruby is the red variety, and Sapphire is the variety that encompasses all other colors, although the most popular and valued color of Sapphire is blue. Sapphire is also only used to describe the gem variety; otherwise it is simply called Corundum.

Corundum is a very hard, tough, and stable mineral. For all practical purposes, it is the hardest mineral after Diamond, making it the second hardest mineral. It is also unaffected by acids and most environments. Translucent brown Corundum and Emery are the most common forms of Corundum. These are fairly common forms, and due to their great hardness and prevalence are the most favorable abrasives. The industrial term "emery" describing Corundum abrasives is derived from the variety Emery which is mined specifically for its use as an abrasive. Erosion may cause Emery to crumble and form sand, which are sometimes called "black sands."

Corundum is easily synthesized, and many Corundum abrasives are synthetic. Synthetic gems are also easily created by adding traces of certain color producing elements to the Corundum solution, and letting the solution solidify into a boule, or synthetic, unprocessed "mineral" with a particular shape. This process is called the Verneuil process.

Natural Ruby and Sapphire gemstones may have their color artificially enhanced or deepened through heat treatment when used as gems. Some dark blue stones from certain localities may also be made a bright blue desirable color.
Chemical Formula Al2O3
Composition Aluminum oxide
Color Many colors, including blue, red, violet, pink, green, yellow, orange, gray, white, colorless, and black. Occasionally also multicolored or striped.
Streak White
Hardness 9
Crystal System Hexagonal
3D Crystal Atlas
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Crystal Forms
and Aggregates
Crystals occur as hexagonally shaped prismatic and tabular crystals, and as bipyramidal hexagons that are wider in the center and taper thinly on the ends. Crystals are usually elongated and striated crosswise, and sometimes occur in thin plates. Crystals are commonly smooth or rounded on the edges due to alluvial action. Also as barrel-shaped hexagonal crystals, modified octahedrons, massive, and as rounded, waterworn alluvial pebbles. Crystals are sometimes striated or etched.
Transparency Transparent to opaque
Specific Gravity 3.9 - 4.1
Luster Vitreous to adamantine
Cleavage None, but commonly exhibits rhombohedral and basal parting
Fracture Uneven
Tenacity Brittle
Other ID Marks 1) May fluoresce orange, yellow, or red in shortwave ultraviolet light.
2) May be triboluminescent.
Complex Tests Insoluble in acids
In Group Oxides; Simple Oxides
Striking Features Immense hardness, high specific gravity, distinctive crystal shape
Environment In altered metamorphic rocks such as marble and hornfels, in nepheline syenite pegmatites, and in placer deposits.
Rock Type Igneous, Sedimentary, Metamorphic
Popularity (1-4) 1
Prevalence (1-3) 2
Demand (1-3) 1


 -  Black, massive variety of Corundum, with color caused by mixture of Magnetite, Hematite, and Spinel.
 -  Orange-pink gem variety of Sapphire. Padparadschah is a very rare and sought after variety.
 -  Pink to red variety of Corundum. See the gemstone section on Ruby for complete details.
 -  Sapphire refers to all gem varieties of Corundum, excluding the red variety of Corundum which is classified as Ruby. In the gem trade, it refers specifically to the blue gem variety of Corundum, unless a color is mentioned as a prefix to sapphire (i.e. Green Sapphire). Sapphire with a color other than blue is often called a "fancy" in the gem trade. See the gemstone section on Sapphire for complete details.
 -  Ruby displaying asterism in the form of a six-rayed star.
 -  Sapphire displaying asterism in the form of a six-rayed star.
 -  Incorrect term in the gem trade occasionally used to describe blue Sapphire.

Ruby and Sapphire are the most famous gemstones after Diamond. They have retained their high value throughout the centuries and are continuously and vigorously mined throughout the world. Red rubies and blue Sapphires are the most popular Corundum gemstones, though other Sapphire colors such as orange, yellow, and green are becoming increasingly popular in gemstone use.

Corundum has some electrical uses, and non gem material, such as Emery, is used as an abrasive because of its high hardness.

The gemstone applications of the Corundum varieties are discussed in greater detail in their own dedicated gemstones pages. Make sure to also see the gemstone section on Ruby and Sapphire.

The area of Mogok, Burma (Myanmar) is the source of some of the best gem-quality Ruby. Another significant Burmese deposit is Mong Hsu, which is currently a major Ruby producer, though its color isn't as fine as the Mogok material. Sri Lanka, especially the area of Ratnapura, Bibile, and Rakwana, is an excellent source of gem quality Corundum of all colors. The finest and largest crystallized specimens and aggregates of gem-quality Sapphire are from Sri Lanka.

Ruby with a brownish tint comes from Thailand, in the Chantaburi District. Fine gem Ruby and Sapphire comes from Luc Yen, Yenbai Province, Vietnam; the Hunza Valley, Gilgit, Pakistan; and Jegdalek, Sorobi District, Afghanistan. Gem quality Sapphire is well-known at Pailin, Cambodia. Translucent to opaque Ruby, sometimes with asterism, is very abundant at Mysore (Mysuru) and Subramanium, in Karnataka, India.

Africa has recently become a significant producer of Corundum, especially in Madagascar, where it is found in the Zazafotsy Quarry, Ambahatraso; and in Andranondambo, Amboasary District. A famous African locality in which opaque rubies are found in green Zoisite is is in Longido, Kilimanjaro Region, Tanzania. Another important Tanzanian Ruby mine is Winza, in Arusha.

In the U.S., the Yogo Gulch in Judith Basin Co. has produced choice, deep blue Sapphire crystals. Not far from the Yogo Gulch, near Helena, waterworn Sapphires stones are found in the Missouri River throughout its length in Lewis and Clark County. Montana is also the claim to a few other localities: the Gallatin Gateway and Salesville, Gallatin Co.; Rock Creek, Granite Co.; and Cottonwood Creek, Deer Lodge Co. In California, well-formed, elongated Corundum crystals were found in Cascade Canyon in the San Gabriel Mountains, San Bernardino Co. A well known Ruby locality is the Cowee Creek District, Macon Co., North Carolina, especially the area of Franklin.

Large deposits of Emery were once worked near Peekskill, Westchester Co., New York; and Chester, Hampden Co., Massachusetts. An important Canadian occurrence of Corundum is the Burgess Mine, Bancroft, Hastings Co., Ontario.

Calcite, Albite, Muscovite, Spinel, Almandine, Kyanite

Can be confused with many minerals, particularly the silicates, but the high hardness and specific gravity of Corundum can easily distinguish it from any mineral.

corundum PHOTOS
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