Ruby is a variety of corundum (aluminium oxide) which is the second hardest natural mineral on Earth behind Diamond. sapphire is also a variety of corundum so ruby and sapphire are scientifically the same mineral with the colour being the only differentiating factor.  Its deep red colour is caused mainly by chromium and only red corundum is entitled to be called ruby, all other colours are classified as sapphire. The similarity between rubies and sapphires has only been known since the beginning of the 19th century, up until then, red garnet and spinels were also thought to be rubies.  The 'Black Ruby' and 'Timur Ruby', both of which are in the British crown jewels, were incorrectly named and both are in fact spinels.  The name spinel comes from the Greek word for 'spark' and refers to its fiery red colour.  The word ruby comes from the Latin 'ruber' meaning 'red'.

Colour is the most important factor in determining the value of fine rubies.  A good coloured stone, one that is pure and brilliant and more than three carats, will command an extraordinary price and is said to be the most valuable of all gems.

It is extremely rare to find a perfect stone and  the vast majority have imperfections of some kind.  This is because millions of years ago when they were being created deep inside the core of the earth, chrome was the element which gave them their deep red colour, but at the same time it was also responsible for causing a multitude of fissures and cracks within the crystals.  Only a few stones were given the right conditions in which to grow undisturbed and to crystallize to perfection.

A significant area for mining during the nineties was a small town in North East Myanmar (formerly Burma).  When first mined, these stones were not expected to be gem grade because they displayed two colours, a purple to black core and a bright red periphery.  Only after it was discovered that the core could be turned to deep red through heat treatment did they begin to increase in value.

Rubies can be found in a number of locations worldwide and about 90% are heated to enhance their colour.  It's usually the rough material that is heated before cutting and an unheated stone is considered to be very unusual.

Ruby is thought to have been one of the gemstones in the high priest's breastplate or breastplate of Aaron.

Graded 9 on Mohs scale of mineral hardness, ruby is the birthstone for the month of July.

Further Reading:
Wikipedia on Ruby



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