Ruby | Variety of the Mineral Corundum



unpolished slice of the mineral red corundum



About the Gemstone Ruby

Ruby is red corundum the second hardest mineral on Earth.  Sapphire which is blue corundum is almost exactly the same stone from a geological perspective with the most significant difference being colour.  Only red corundum is entitled to be called ruby whilst all other colours are known as sapphire.  The presence of chromium is the primary cause for the stone's distinctive red colour.

The similarity between rubies and sapphires has only really been understood since the beginning of the 19th century and until then, red garnets and spinels were also believed to be rubies.  The Black Prince's Ruby and Timur Ruby both incorrectly identified were later found to be spinels.  Spinel comes from the Greek word for spark and is a reference to the gemstone's fiery red colour whilst ruby originates from the Latin word 'ruber' meaning 'red'.

Colour is the most important factor in determining the value of a ruby and the finest grade gemstones command extraordinary prices.  It is however very rare to find a perfect stone and the vast majority of rubies have imperfections or anomalies of some kind.  Millions of years ago whilst growing deep within the core of the earth, the chromium responsible for the stone's distinctive colour also caused a multitude of fissures and cracks within the crystals.  Only a small number of rubies were given exactly the right conditions in which to grow undisturbed and to crystallize to perfection.

An important mining area during the 1990's was a small town in North East Myanmar (formerly Burma).  Rubies that were initially discovered at this location were not expected to be classified as gem grade because they displayed two colours, a purple to black core and bright red periphery.  It was only after it was discovered that the colour of the core could be changed to a deeper shade of red by heating the ruby that its value increased dramatically.

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

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

Ruby which is red corundum is graded 9 on Mohs scale of mineral hardness.  It's the birthstone for the month of July with onyx being an alternative.



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Further Reading

Information from Wikipedia