Ruby is the gemstone variety of the mineral corundum which is the second hardest naturally occurring mineral on Earth behind diamond. Sapphire which is also a variety of corundum is scientifically the same stone with colour being the only differentiating factor.  Only red corundum is entitled to be called ruby whilst all other colours are known as sapphire.  The deep red colouration in rubies is caused primarily by the presence of chromium.

The similarity between rubies and sapphires has only been known since the beginning of the 19th century and up until then, red coloured garnets and also spinels were believed to be rubies.  The Black Prince's Ruby and Timur Ruby were incorrectly named and both are in fact spinels. The name spinel comes from the Greek word for 'spark' and is a reference to the stone's fiery red colour.  Ruby originates from the Latin 'ruber' meaning 'red'.

Colour is the most important factor in determining the value of a fine ruby and fine grade stones can command extraordinary prices.  It is however very rare to find a perfect stone and the vast majority have imperfections or anomalies of some kind.  Millions of years ago when rubies were being created deep inside the core of the earth the chromium that causes their distinctive colour was also responsible for a multitude of fissures and cracks within the crystals.  Only a small number of stones were given the right conditions in which to grow undisturbed and to crystallize to perfection.

A significant area for mining rubies during the 1990's was a small town in North East Myanmar (formerly Burma).  When first mined the stones were not expected to be classified as gem grade because they displayed two colours, a purple to black core and a bright red periphery.  Only after it was discovered that the colour of the core could be changed to a deeper shade of red by heating the stone did the value of the rubies increase.

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

Ruby is believed to have been one of the gemstones in the breastplate of the high priest also known as Breastplate of Aaron.  This religious garment was worn during biblical times by the Jewish high priest when presenting himself to God. 

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



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Further Reading:
Wikipedia on Ruby