A Perfect Ruby is Extremely Rare
Ruby is a variety of red corundum which is the second hardest mineral known to man. Sapphire which is blue corundum is from a geological perspective almost exactly the same stone 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 of ruby's distinctive red colour.
The similarity between rubies and sapphires has only really been fully understood since the beginning of the 19th century. 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 ruby and the vast majority of stones have minor imperfections or anomalies. Millions of years ago whilst growing deep within the core of the earth, the chromium responsible for the red colour also caused a multitude of fissures and cracks within the crystals. Only a small number of rubies were given exactly the right conditions 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 here 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 some time before it was discovered that by heating the ruby the colour of the core could be changed to a deeper shade of red and by doing that the value of the stones increased dramatically.
Ruby can be found in a number of locations worldwide and about 90% are heat treated to enhance or change their colour. It's usually the rough stone that's heated before cutting begins. An unheated ruby is 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 Aaron the first Jewish high priest whilst presenting himself to God.
Red corundum which grades 9 on Mohs scale of mineral hardness is the birthstone for the month of July. On the traditional birthstone chart the gemstone onyx is used as an alternative.
The red corundum featured in the photo at the top of this article is from Madagascar. Photo courtesy of Stan Celestian. The image is clickable and redirects to the original full size photo.