Rough Ruby (Corundum) in Alabaster

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.


Corundum Pictures

Ruby (Corundum)

Chemical Composition :

Aluminium Oxide – Al2O3 (trace elements Chromium, Iron, Titanium, etc. are responsible for the different colored varieties.

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.

Cuts & Uses :

Facetted cuts (usually mixed cuts), cabochons, beads, carvings, etc.

Dispersion : 0.018
Hardness : 9
Lustre : Vitreous
Optic Character :

Anisotropic, D.R., Uniaxial negative

Pleochroism : Strong in deep colored varieties
Refractive Index / Birefringence :

1.762 – 1.770 / 0.008; Range: 1.754 – 1.778

Synthesis :

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