About
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.
Crystal Forms & Aggregates
Other ID Marks
2) May be triboluminescent.
Complex Tests
Striking Features
Environment
Varieties
✓ common · ✗ uncommon
- Emery— Black, massive variety of Corundum, with color caused by mixture of Magnetite, Hematite, and Spinel.
- Padparadschah— Orange-pink gem variety of Sapphire. Padparadschah is a very rare and sought after variety.
- Ruby— Pink to red variety of Corundum. See the gemstone section on Ruby for complete details.
- Sapphire— 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.
- Star Ruby— Ruby displaying asterism in the form of a six-rayed star.
- Star Sapphire— Sapphire displaying asterism in the form of a six-rayed star.
- Ultralite— Incorrect term in the gem trade occasionally used to describe blue Sapphire.
Uses
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.
Noteworthy Localities
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.
Common Mineral Associations
Distinguishing Similar Minerals
Photos
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