CORUNDUM

The mineral called corundum forms as precipitous bipyramidal, barell shaped prismatic, flat tabular or rhombohedral crystals. It also occurs in massive and granular habits. Corundum is the mineral name which is used under the mineral groups. Most people are familier with corundum, however very few know it by its mineral name.

A gemstone quality specimen of corundum with a deep red colour is known as ‘Ruby’. A gemstone quality corundum with pinkish-orange colour is called ‘Padparadscha’. A gemstone quality corundum with any other colour or colourless is called ‘Sapphire’. ex. Blue, Pink, Yellow, Violet, Purple, Green, Brown, Black etc. and also bi-colour sapphire. Gemstones in rough form with the appearance of silky, Milky, Smoky etc. are called ‘Geuda’. This variety could be converted into Blue, Yellow etc., by heat treatment.

Corundum is also a rock forming material. It forms in silica-poor igneous rocks and metamophic rocks, rich in aluminium. It is an insoluble material.

Most corundum are found in alluvial deposits as secondary gem deposits. These alluvial deposits are sources of rubies, sapphires in several parts of the world. Ex. Sri Lanka, Myanmar, Cambodia, India, Afghanistan, Thailand etc. Corundum are found as a primary mineral in igneous rocks such as syenite, nephline in locations where aluminous shales or bouxites have been exposed to contact metamorphism. Recent discovered corundum mineral deposit found in Sri Lanka in Kataragama area as blue sapphire crystals.

Corundum is very hard. It serves as the index mineral with a hardness of 9 on the mohs hardness scale. It is the 3rd hardest mineral known, with Diamond and Moissanite being the only other minerals with a greater hardness. This high hardness makes corundum specially useful as an abrasive. Crushed corundum is screened to produce uniformly-sized grits and powders. These are used as grinding media and used to manufacture polishing compounds, sand papers, grinding wheels and cutting tools.

Physical properties of corundum are as follows..

Lustre – Adamantine, Vitreous, Pearly

Diaphaneity – Transparent, Translucent, Opeque

Colours – As Ruby, Padparadscha & Sapphire

Streak – White

Hardness – (Mohs)-9

Tenecity – Brittle

Cleavage – Non observed

Parting – Rhombohedral and Basel parting

Fracture – Irregular/Uneven, Conchoidal

Density – 3.98-4.1 (Measured)

3.997 g/cm3 (calculated)

Important optical property of corundum is refractive index- 1.762 to 1.770 and maximum Birefringence of 0.009

Asterism is a special optical effect in corundum. It shows 06 rays or sometimes 12 rays which is rare. Those are called Star Ruby or Star Sapphire.

Basic chemical composition of corundum is Aluminium Oxide (A12 O3) i.e. Oxide of Aluminum with an Aluminum percentage of 53.2 and Oxygen 46.8. In its pure form it is colourless and called White Sapphire, but, when a small percentage of impurities such as Chromium, Titenium, Iron and rutile is present, it produces different colours and appearance.

As the presence of different impurities in a corundum, it could be distinguished into 04 categories as follows:

Ruby – Ruby forms as bipyramidal, prismatic, flat tabular or Rhombohedral crystals. It is red in colour due to the presence of chromium. To appear as deep red ruby colour chromium percentage should be 0.10. Colours of Ruby vary as Pigeon Blood Red, Golden Red, Bright Red, Pinkish Red, and Pastel Red. These colours of Ruby are used as trade names.

Padparadscha – This Corundum variety mostly forms as bipyramidal and prismatic crystals. It is pinkish orange in colour due to the presence of chromium and iron as impurities.

Sapphire – Different colours of corundums excluding ruby & padparadscha are called sapphires. Colours vary as mentioned above. Impurities are Titanium, Iron, Chromium

Geuda – This is a special variety of corundum which shows dull appearance or translucent until   convert into colourfull gemstone by heat treatment. It looks like smoky, milky, silky and some are with diesel colour inside the stones. Due to the presence of Rutile (Ti O2) as impurities, it appears as described above.