Corundum

Corundum is a crystalline form of aluminium oxide (Al2O3) with traces of iron, titanium and chromium.[1] 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 and padparadscha if a pink-orange, while all other colors are called sapphire.

The name "corundum" is derived from the Tamil word "kuruntam" meaning "ruby", and related to Sanskrit "kuruvinda".

Because of corundum's hardness (pure corundum is defined to have 9.0 Mohs), it can scratch almost every 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 an average hardness near 8.0.

Source: en.wikipedia.org/wiki/Corundum

Corundum Pictures

Ruby (Corundum)

Corundum Properties

Chemical Composition: 
Aluminium Oxide - Al2O3 (trace elements Chromium, Iron, Titanium, etc. are responsible for the different colored varieties.
Colors / Varieties: 
  • Ruby: Red / Pink
  • Sapphire (all colors)
    • White Sapphire
    • Blue Sapphire
    • Pink Sapphire
    • Green Sapphire
    • Yellow Sapphire
    • Pink-Orange Sapphire (Padparadscha)
  • Phenomenal
    • Color Changing Sapphire
    • 6-Ray Star Ruby / 6-Ray Star Sapphire
    • Parti-colored Sapphire
    • Trapiche Ruby / Sapphire
Crystal System / Forms: 
Trigonal System / Variable: Generally pseudo-hexagonal prisms with either pyramidal and / or pinacoidal terminations. In some cases rhombohedral faces are also observed. Barrel shaped habits, tabular and rhombohedral habits are common.
Hardness: 
9
Specific Gravity: 
3.98 - 4.00
Optic Character: 
Anisotropic, D.R., Uniaxial negative
Lustre: 
Vitreous
Refractive Index / Birefringence: 
1.762 - 1.770 / 0.008; Range: 1.754 - 1.778
Pleochroism: 
Strong in deep colored varieties
Dispersion: 
0.018
Geological Occurrence: 
In igneous and metamorphic rocks (marble, dolomite, etc.)
Cuts & Uses: 
Facetted cuts (usually mixed cuts), cabochons, beads, carvings, etc.