Corundum

Category  :  Mineral
Hardness  :  9
Gravity  :  3.95-4.1
Reflection  :  1.77 / 1.76
Color  :  Brown to gray, less often red, blue, white,or yellow.
Chemical formula  :  Aluminium oxide, Al2O3
Crystal system  :  Trigonal
Major varieties  :  Ruby, Sapphire, Pink Sapphire, Violet Sapphire, Yellow Sapphire, Green Sapphire, Colourless Sapphire, Star Rubies and Sapphires. Different Coloured Asteriated

The name corundum probably derived from the Sanskrit "kuruvinda" meaning "ruby". In India a pinkish-orange sapphire is called padparadscha and a pink light-red sapphire is called patmaraga.

Due to corundum's hardness (pure corundum is defined to have 9.0 Mohs), it can scratch almost every other mineral, leaving behind a streak of white on the other mineral. It is commonly used as an abrasive, on everything from sandpaper to large machines used in machining metals, plastics and wood. Some emery is a mix of corundum and other substances, and the mix is less abrasive, with a lower average Mohs hardness near 8.0.

In addition to its hardness, corundum is unusual for its high density of 4.02 g/cm³, which is very high for a transparent mineral composed of the low atomic mass elements aluminium and oxygen.

Corundum occurs as a mineral in mica schist, gneiss, and some marbles in metamorphic terranes. It also occurs in low silica igneous syenite and nepheline syenite intrusives. Other occurrences are as masses adjacent to ultramafic intrusives, associated with lamprophyre dikes and as large crystals in pegmatites. Because of its hardness and resistance to weathering, it commonly occurs as a detrital mineral in stream and beach sands. The largest documented single crystal of corundum measured ~65x40x40 cm3.

Corundum for abrasives is mined in Zimbabwe, Russia, and India. Historically it was mined from deposits associated with dunites in North Carolina and from a nepheline syenite in Craigmont, Ontario. Emery grade corundum is found on the Greek island of Naxos and near Peekskill, New York. Abrasive corundum is synthetically manufactured from bauxite.

Ruby

A ruby is a pink to blood-red gemstone, a variety of the mineral corundum (aluminum oxide). The red color is caused mainly by the presence of the element chromium. Its name comes from rubber, Latin for red. Other varieties of gem-quality corundum are called sapphires. The ruby is considered one of the four precious stones, together with the sapphire, the emerald, and the diamond.

Blue Sapphire

Color in gemstones breaks down into three components: hue, saturation, and tone. Hue is most commonly understood as the "color" of the gemstone. Saturation refers to the vividness or brightness or "colorfulness" of the hue, and tone is the lightness to darkness of the hue. [3] Blue sapphire exists in various mixtures of its primary and secondary hues, various tonal levels (shades) and at various levels of saturation (brightness): the primary hue must, of course, be blue.

Pink Sapphire

This is the name given to the pink variety of corundum. Pink sapphire and ruby are regarded as 2 different varieties, despite the fact that the only difference is their depth in color. Pink sapphire’s most striking characteristics is its luster, common to all corundum and most noticeable in light colored specimens. Like all forms of corundum pink sapphire has a very good luster. It is normally given a mixed, oval cut and sometimes has fine inclusions and liquid veils in lacelike formations.

Violet Sapphire

It is formally known as “oriental amethyst” on account of its color. When the color of the sapphire tends to be blue-violet, red violet or pink violet, it is fairly distinctive, but not so when it is a true violet or violet with a slight hint of red. Violet sapphires are easily distinguished, however, by their physical characteristics. If the density is measured with a heavy liquid such as methylene iodide, corundum rapidly sinks, while amethystine quartz floats.

Yellow Sapphire

Yellow sapphire may occur in quite a wide range of colors, from pale to canary yellow, gold, honey, and brownish yellow .The lighter, brighter colors are the most common. Medium-size or large stones are often seen, generally with a mixed ova cut having a rather large pavilion (to increase a depth of color).

Green Sapphire

In the 19th century green sapphire was known as “oriental emerald”. Due to its iron content, green sapphire is generally quite a strong, bright green color, sometimes with green to bluish green or yellowish green pleochroism. Green sapphire comes mainly from Australia but it is also found in the United States and Thailand.

Colourless Sapphire

When the crystal structure of corundum does not contain trace elements that act as colorants, it is completely clear, although this form is the least known in the gem market. It is perfectly colorless or occasionally has a slight yellow tinge and has the typical luster of corundum. It is given a round (or almost round) mixed cut, or a slightly modified brilliant cut, having mainly been used as a substitute for diamond.

Star Rubies and Sapphires

The most striking phenomenon of rubies and sapphires is the development of the six ray star, arranged in perfect symmetry, which shifts its center as the stone is moved. It is clearly visible under a single light source such as the sun or a lamp. Such stones are invariably given a round or oval cabochon cut.

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