CORUNDUM

•Next to diamond, varities of corundum are given importance in the gem trade.•Chemical composition -----Al2O3.•Crystal system ----- Hexagonal.•Moh’s Scale Hardness----9 •Melting point ----- High, 2000-20500c•Termal conductivity ----- High •Lustre---- vitreous to adamantine.•Chemically inert. Not easily acted by any acids.•Ruby and Sapphire are the most important varieties of corundum.

Ruby is the name given to red coloured corundum. All other colour varieties including colourless sapphire.

Traditionally Ruby is considered as “Raja Ratna” (king of gems).

Best shades of colour is compared to blood red of pigeon.

By and large, there is a tendency to increase in flows as colour deepens.

CORUNDUM

RUBY

Rough stones are flattened.flowless deep coloured Ruby of 5 carats(1g) or more are exceedingly rare.

Fine quality less than 1 carat are frequent and prices are affordable.

Red colouration of ruby ---- due to Cr2O3 impurity, allochromatic

Colour is pale (pink)----- low concentration of Cr2O3

CORUNDUM

RUBY

Colour deepens ----1-4% of Cr2O3.

Presence of Fe in Ruby reduces richness of red colour.

Zoning is common in Ruby. Rutile and Zircon needles in Ruby----

undesirable pattern of silk.

CORUNDUM

RUBY

finger prints & growth zoning in natural ruby

All the varieties of corundum except blood red colour, including uncoloured ones, are called “sapphire”.

Occurs in several colours Old term such as Oriental Topaz (Yellow Sapphire) Oriental Emerald (Green Sapphire) Oriental Amethyst (violet Sapphire)

are still in use.

SAPPHIRE

CORUNDUM

Most Indians believe yellow sapphire------Topaz. Sapphire is an allochromatis mineral, and cause of colour, is

mainly due to Fe, Ti, Cr and V Fe+2—Uncoloured or pale green Fe+3------Yellow Fe+2+Ti---Blue Fe+3+Ti---Pale yellow Fe+2+Cr+Ti----Violet Fe+3+Cr---Orange V+Cr---Greenish blue Of all gem minerals, maximum experiments have been carried

out on varieties of corundum for colour enhancements, especially Sapphire.

CORUNDUM

SAPPHIRE

Most blue and yellow Sapphire sold are colour enhanced.

Secondary colours are imparted to mineral by heat treatment and by irradiation in gamma rays chamber.

Such colours, however bleached on exposure to strong sunlight. Heat treated ones are stable to some extent.

Apart from Ruby, the most valuable variety of corundum is “blue sapphire”- Neel.

CORUNDUM

SAPPHIRE

This beautiful stone unfortunately is not popular in India. Many Indians are afraid to wear this stone as it may bring “bad Luck”.

Common corundum is fairly abundant in nature, but not gem corundum.

Occurs in silica poor rocks, like nepheline syenites, alkali igneous rocks.

CORUNDUM

occurrence

It also occurs as veins and segregations associated with peridotites.

It may also occur in contact aureoles in thermally altered rich sediments. (shales and limestones).

CORUNDUM

occurrence