corundum

CORUNDUM

Chemical Formula	Al₂O₃
Composition	Aluminium oxide
Colour	Many colours, including blue, red, violet, pink, green, yellow, orange, gray, white, black and colourless
Hardness	9
Specific Gravity	3.9 - 4.1
Transparency	Transparent to translucent to opaque
Crystal Forms and Aggregates	(Hexagonal) Crystals occur as prismatic and flat hexagons. Crystals are many times striated crosswise and elongated, and sometimes occurs in thin plates. Also occurs massive and as rounded, waterworn pebbles.
Luster	Vitreous to Sub-Adamantine
Cleavage	None
Fracture	Uneven to conchoidal
Toughness	Excellent; Brittle, if repeated twinning present
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, Padparadscha, 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. In gem terms, it refers to the blue variety of Corundum, unless a colour is mentioned as a prefix to sapphire (i.e. Pink Sapphire). Sapphire with a colour 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. Padparadscha - Orangish-pink variety of Corundum Nowadays, all sapphire is classified by its colours (i.e. pink colour Sapphire is "Pink 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 colour. 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 - Colourless, 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.
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 colour 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 colour.
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.
Striking Features	Immense hardness, high specific gravity, crystal shape
Complex Tests	Insoluble in acids
Geographical Origin	The area near Myanmar (Burma), Mogok, is the source of many gem quality rubies and some Sapphires. Sri Lanka/Ceylon, especially the area around Ratnapura and Elahera, are an excellent source of gem quality Corundum of all colours. Sri Lanka's mines have been worked down the centuries and still continue today to produce high volumes of good qualitiy corundum. Blue colours, and stones of all other colours come from these mines.The highly prized pink/orange Padparadscha stones are also found in Sri Lanka, as well as blue colour change sapphires. Rubies with a brownish tint come from Thailand, in the Chanthaburi District. India, East Africa, Madagascar, Brazil, and Afghanistan have also produced fine gem material.

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