Better known to the public as sapphire and ruby, the mineral species corundum is a widely distributed oxide of aluminum which ranks next to diamond in hardness and sometimes occurs so abundantly that it is mined for abrasive purposes. However, clear gemstone varieties are far less common than the course abrasive types, one of which “emery” is familiar to craftsmen as the coating on emery paper and abrasive wheels. In North America, much abrasive corundum has been mined from deposits in Canada and the Eastern part of the US, while clear gem quality material has been obtained in large quantity from Montana.
Some specimens exhibit the beautiful effect known as “asterism”, forming the ever popular star sapphire and star ruby.
By custom, the individual species name ruby is applied only to gem material of pure intense red, all other colors being sapphire. Consequently it is accurate to address green, yellow, blue, purple and even colorless corundum as being sapphire.
Another variety much less translucent than gem grades, is known as “adamantine spar”, and is noted chiefly for its bronzy brown chatoyancy. Emery is merely extremely impure massive corundum in which a considerable amount of black magnetic oxide from iron is included, namely “magnetite”.
The name corundum is a modified form of the “Sanskrit “word of unknown original significance except for its application to this mineral species, while sapphire is derived from the ancient Greek word for blue. It appears that the latter term may have been applied originally to another blue gemstone, most likely lapis-lazuli, and was later attached to the blue variety of corundum.
Ruby receives its name from the Latin rubber, meaning “red”. Like sapphire, this term was also applied indiscriminately to any hard transparent red gemstone and was only attached to red corundum within the last several hundred years. When corundum gems were introduced into Europe and England during the medieval period, their exact identity was unknown and they were frequently confused with the mineral species spinel which occurs in a similar range of colors in the gem deposits of the Far East.
In composition, corundum is aluminum oxide; if it were to be pure it would be colorless. To the delight of man this is seldom the case, slight traces of iron, chromium, titanium, and other metallic elements convey grand colors which make clear specimens prized as gems. The presence of chromium imparts the beautiful red of ruby while the blue of sapphire is caused by titanium and iron together. Chromium combined with oxygen as chromic oxide fits neatly into the crystal structure of corundum and therefore tends to show a uniform coloration to ruby.
On the other hand, titanium, iron, and oxygen combine to form the mineral “ilmentite” which is not capable of fitting itself so nicely into the crystal lattice; this tends to collect in clouds, patches or areas and imparting corresponding irregularities in coloration. In a similar manner, other impurities give rise to uneven color distribution, somehow causing several bands or patches of contrasting hue to appear within the same crystal.