Corundum

Corundum is a crystalline form of aluminium oxide (Al2O3) with traces of iron, titanium and chromium.It is a rock-forming mineral. It is one of the naturally clear transparent materials, but can have different colors when impurities are present.  Transparent specimens are used as gems, called ruby if red, while all other colors are called sapphire.

   The name "corundum" is derived from the Tamil word "kuruntam" meaning "ruby".

Main Characteristic

Chemical composition: Al2O3

Harness:9.0

Refractive index: 1.762-1.770

Density: 4.00(±0.05)g/cm³

Dispersion: 0.018

Transparency: Transparent to subtransparent

Luster：vitreous luster to dull luster

The deposit area of corundum.

Corundum occurs as a mineral in mica schist, gneiss, and some marbles in metamorphic terranes. intrusives. Because of its hardness and resistance to weathering, it commonly occurs as a detrital mineral的in stream and beach sands .The largest documented single crystal of corundum measured about 65×40×40 cm 。

The most famous high grade corudum( ruby and sappire )supplier countries are Burma,Sri lanka,Tailand,Vietnam,and Cambodia. Other supplier countries are China,Australia ,US as well as Tanzania.

Ornamental Attribute of Corundum

Ruby:
 legendary,the ruby would provide the wearer with wisdom,health,happy love.Once upon a time ,when people wearing a ruby diamond on the left hand or wearing plastron with ruby on the left side ,he would avoid conflicts or evil things. today ,people still regard ruby as the gems ,the birthstone of glorious July,the commemorate gift of 40th wedding anniversary.

Sappire :

This may a day ,people consider the sappire as the symbol of honesty and a great person who enjoy high prestige and command universal respect.And sappire is named as the birthstone of September and the commemorate gift of 45th wedding anniversary.

In ancient China, we can see the official cap showing three ranks by button of sappire on top.

In the western counties , the Christian engrave the Ten commandments on the sappire so that everyone could see the treasure of church .

Because corundum's hardness (pure corundum is defined to have 9.0 Mohs), it can scratch almost every other mineral. So apart from ornamental uses ,synthetic corundum is also used to produce mechanical parts (tubes, rods, bearings, and other machined parts), scratch-resistant optics, scratch-resistant watch crystals, instrument windows for satellites and spacecraft  (because of its transparency from the UV to IR), and laser components.

In additions ,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 hardness near 8.0.

Synthetic corundum

In 1837, Gaudin made the first synthetic rubies by fusing alumina at a high temperature with a small amount of chromium as a pigment.

In 1903, Verneuil announced he could produce synthetic rubies on a commercial scale using this flame fusion process.And they producted the synthetic sappine in the year of 1910 in the same processing method.

In recent decades ,The Verneuil process allows the production of flawless single-crystal sapphires, rubies and other corundum gems of much larger size than normally found in nature.  

It is also possible to grow gem-quality synthetic corundum by flux-growth and hydrothermal synthesis. Because of the simplicity of the methods involved in corundum synthesis, large quantities of these crystals have become available on the market causing a significant reduction of price in recent years.  

But as a kind of jewelry ,the ruby and sappire show an increasing demand in the markets in the recent years.

Quality assessment of corundum

To evaluate the quality of corundum ,the following factors should be taken into considerations :