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Chemical Formula | Al2O3 |
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Composition | Aluminum oxide |
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Color | Many colors, including blue, red, violet, pink, green, yellow, orange, gray, white, colorless, and black |
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Streak | White |
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Hardness | 9 |
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Crystal Forms and Aggregates |
(Hexagonal) Crystals occur as prismatic and flat hexagons, as well as bipyramidal hexagons. Crystals are many times elongated and striated crosswise, and sometimes occurs in thin plates. Also occurs massive and as rounded, waterworn pebbles. |
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Transparency | Transparent to translucent on thin edges |
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Specific Gravity | 3.9 - 4.1 |
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Luster | Vitreous to adamantine |
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Cleavage | None, but commonly exhibits rhombohedral and basal parting |
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Fracture | Uneven |
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Tenacity | Brittle |
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Other ID Marks | 1) Many specimens fluoresce orange, yellow, or red in shortwave
ultraviolet light 2) Many specimens are triboluminescent |
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Varieties | Note: Every variety of Corundum is
classified and is always called by its variety name. The
term "Corundum", when used alone, refers to a
brown, non-transparent variety. All other varieties
(Sapphire, Ruby, Padparadschah, and Emery) are always
known by their respective names. Sapphire - In scientific terms, the name Sapphire
refers to all varieties of Corundum excluding Ruby
and Emery (see below). In gem terms, it refers
to the blue 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. Ruby - Red variety of Corundum. Emery
- Black, massive variety of Corundum, with color caused by
mixture of Magnetite, Hematite, and Spinel. Padparadschah
- Orange-pink variety of Sapphire.Nowadays, all sapphire is classified by its colors (i.e. green color Sapphire is "Green Sapphire"). However, the "oriental" prefixes were used in the past do describe a sapphire of a particular color in reference to a particular gem of that color. Below is a list of all the "oriental" sapphires: Oriental Topaz - straw yellow, gem quality Sapphire Oriental Emerald - Light to dark green, gem quality Sapphire Oriental Amethyst - Violet to pink, gem quality Sapphire Oriental Peridot - Yellow-green, gem quality Sapphire Oriental White Sapphire - Colorless, gem quality Sapphire Sapphire (mentioned singularly on the gem market) - Blue Sapphire Ultralite - Incorrect term in the gem trade sometimes used to describe blue Sapphire Star Sapphire - Sapphire displaying asterism. Ruby may also display asterism, in which it is Star Ruby. |
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In Group | Oxides ; Simple Oxides |
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All About | Corundum is a very hard, tough, and stable mineral. 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 and when ground up, they are the most favorable abrasives. The industrial term "emery" describing Corundum abrasives is derived from the variety Emery which is mined for abrasive use. Erosion may cause Emery to crumble and form sand, 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 stones are commonly heated to intensify the color. |
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Uses | Ruby and Sapphire are perhaps the most
famous gemstones. They have been mined for thousands of
years and were regarded as the most respectable
gemstones. They retained their high status and are now
commercially mined like never before. Not only are the
Red rubies and blue Sapphires mined as gemstones, but
other colors are also elite gemstones and are mined for
gem use. Corundum has some electrical uses, and non gem quality material, such as emery, is extensively used as an abrasive because of its high hardness. Also see the gemstone section on Ruby Also see the gemstone section on Sapphire |
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Striking Features | Immense hardness, high specific gravity, crystal shape |
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Complex Tests | Insoluble in acids |
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Popularity (1-4) | 1 |
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Prevalence (1-3) | 2 |
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Demand (1-3) | 1 |
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Distinguishing Similar Minerals |
Can be confused with many minerals, particularly the silicates, but its high hardness and specific gravity can easily distinguish it from any mineral. |
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Commonly Occurs With |
Albite, Muscovite, Spinel, Almandine, Kyanite |
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Noteworthy Localities |
The area near Mogok, Myanmar (Burma), is the
source of many gem quality rubies and some Sapphires. Sri
Lanka, especially the area around Ratnapura, is an
excellent source of gem quality Corundum of all colors.
The region along the Umba River in Tanzania has produced
fine Ruby crystals embedded in green Zoisite. Rubies with
a brownish tint come from Thailand, in the Chantaburi
District. India, Madagascar, Brazil, and Afghanistan have
also produced fine gem material. 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: Salesville, Gallatin Co., Rock Creek, Granite Co., and Cottonwood Creek, Deer Lodge Co. Other famous occurrences are at Hastings Co., Ontario; Corundum Hill, near Unionville, Chester Co., Pennsylvania; and several scattered areas in southern California and North Carolina. Large deposits of Emery were worked near Peekskill, Westchester Co., New York. Rubies can be found in the U.S. in the Cowee Creek District, Macon Co., North Carolina. A few scattered finds were reported in Sparta and Ogdensburg, Sussex Co., New Jersey. |
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