Altough the name Corundum is not very known to most people, everybody knows very well the two varietes that belong to this family: Ruby and Saphire. We will first review some common properties of corundum before looking in detail at rubies and sapphires.
Corundum is composed out of two rather common chemical elements Aluminum and Oxygen.
Corundum is the hardest mineral after diamond, but still it is 140 times less hard than diamond, but 7 times harder than topaz. The hardness of corundum can also vary according to the origin or the color of the stone. Sapphire is known to be harder than ruby, and sapphires from Myanmar or Sri Lanka are harder than sapphires from Kashmir. Although corundum is rather hard, it is also quite brittle and one should be careful not to drop it on a hard surface.
The name Sapphire is derived from Greek word for blue. Until the middle ages the term “sapphire” indicated lapis lazuli. Only round 1800 it was discovered that ruby and sapphire both belonged to the group of corundum. From this point on “sapphire” was used to describe the variety of corundum. Other colors got rather confusing names that no longer used today, like “Orient peridot” for green sapphire or “Orient topaz ” for yellow sapphire.
Sapphire is a term applied to ALL colors of corundum except the medium light to dark tone of red to purple red. The name associated with sapphire trade grades are not as clearly based as the ruby on the sources output. We use a color definition such as yellow, blue, pink, etc… for describing a particular sapphire since the word sapphire used alone imparts to the consumers and jewelers the color blue. Orange (salmon) colored sapphire from Sri Lanka is called “ Padparadcha ” (from Singhalese “Lotus flower”), and colorless sapphire is sometimes called ” Leuco-sapphire ”
The main suorces are: Burma, Thailand, Cambodia, Montana (USA), Tanzania..
The blue color of sapphire is caused by the presence of iron and Titanium as chemical impurities in the lattice.The amount of impurities determines the intensity of the color. Most wanted colors are:
- Marine blue
- Royal blue (deep cobalt blue)
- Cornflower blue (top-color deep blue with a touch of violet, also called cashmere blue
- Yellow sapphire is colored by Iron atoms replacing the aluminum atoms in the lattice.
- Green sapphire is colored by Iron
- Violet sapphire, pink or orange sapphires are colored by Chromium and Iron
Only the transparent medium light to dark tones of red to purple red of the mineral corundum can be defined and called ruby. Stones that are light to very light in tone are called pink sapphire. Corundum is found in various places called pink on the earth surface but only under ideal conditions the transparent gem material will be formed. This transparent gem material and the special asteriated (star stone) can be classified as gemstone.
The main sources of the corundum are generally used to designate the various trade names of the ruby. The main sources are: Burma, Thailand, Sri Lanka and Africa.
The name ruby is derived from Latin “rubber”, which means red. Before 1800., spinel and garnet were also considered to be rubies. Only after this date ruby and saphire were recognised to belong to the corundum family.
For this reason some very famous rubies are actually not ruby at all. The “Black Prince’s Ruby” and the “Timor Ruby” in the British crown jewels are not rubies, but spinel.
Large rubies are extremely rare. The biggest ruby of gem quality ever found in Burma and weighted 400 ct. Other famous rubies are the Edward’s Ruby (167 ct, in the British Museum of Natural History, London), Reeves’ star ruby (137 ct, in the Smithsonian Museum, Washington)
The color in ruby is caused by traces of chromic oxide. The color varies from faint pink to dark violet (red).
The famous and most desirable color is “pigeon blood red”, a deep red with a light touch of violet, and this rubies can be found in Burma.
Ruby from Sri Lanka has a lighter red to pink color.
Thailand ruby has a red-brown or violet color.