Two color varities of corundum are used for making jewelry, the red Ruby and the blue sapphire which comprises all other colors.
Common corundums, those not of gemston quality, serves as cutting and polishing material. The well-known polishing material emery is mainly fine-grain corundum, to which magnetite, hematite and quartz are added. The name corundum has its origine in India and probably referred to ruby.
Ruby is thus named because of its red color. It was not until about 1800 that ruby, as well as sapphire, was recognized as belonging to the corundum species. Before that date, red spinel and the red garnet were also designated as ruby.
The red color varies within each individual deposit, so it is not possible to determine the source area from the color. The designations "Burma Ruby" or "Siam Ruby" are therefore strictly erroneous and refer more to quality and origin. The most desirable color is the so-called "Pigeon's Blood", pure red with a hint of blue. The distribution of color is often uneven, in stripes or spots. The substance that provides the color is chromium and in the case of brunish tones, iron is present as well.
As a rough stone, ruby appears dull and greasy, but when cut, the luster can approach that of diamond. Heat treatment is commonly used to improve the color.
Ruby is the hardest mineral after diamond. Ruby has common inclusions. They are not always indicative of lower quality, but show the difference between a natural and synthetic stone.The type of inclusion often indicates the source area.
Deposits: The host rocks of ruby are metamorphic dolomite marble, gneiss and anphibolite. Some of the most important deposits are in Myanmar, Thailand, Sri Lanka and Tanzania. For centuries, the most important have been in upper Myanmar near Mogok. Rubies from Thailand often have a brown or violet tint to them. They are found southeast of Bangkok in the districity of Chantaburi.
Other mining deposits are in Afghanistan, Australia, Brazil, India, Cambodia, Kenya, Madagascar, Malawi, Nepal, Pakistan, Zimbabwe, Tajikistan, Vietnam and the United States. Small ruby deposits can also be found in Switzerland, Norway and on the Soutwest Coast of Greenland.
Famous Rubies: Ruby is one of the most expensive gems, large rubies being rarer than comparable diamonds. The largest cuttable ruby weighed 400 ct; It was found in Burma and divided into three parts. Famous stones of exceptional beauty are the Edwardes ruby (167 ct) in the British Museum of Natural History in London, the Rosser Reeves star ruby (138.7 ct) in the Smithsonian Institution in Washington D.C.
Many rubies comprise important parts of royal insignia and other famous jewerly. The Bohemian St. Wenzel's crown, holds a nonfaceted ruby of about 250 ct. But some gems, thought to be rubies, have been revealed as spinels, such as the "Black Prince's Ruby" in the English state crown and the "Timur Ruby" in a necklace among the English crown jewels.
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U.S. Geological Survey (annual). Gemstones: ch. in Minerals Yearbook, U.S.G.s., Restone, VA, U.S.