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Cabouchon Sapphire
An Attractive Cabouchon Sapphire

Sapphire
Sapphire is the usual name for transparent corundum. On this page, we will often use the word corundum to include sapphire and ruby, because of their shared characteristics.
The usual colour associated with sapphire is blue, but sapphire can be almost any colour.

Corundum
Corundum is the mineralogical name for aluminium oxide.

Colours
Corundum can be colourless, red, pink, blue, black, brown, orange, yellow, green, indigo, violet, or mauve. Red corundum and most pink corundum is called ruby, blue corundum is called sapphire, and other colours are also called sapphire, usually with the colour specified as a prefix to the word sapphire, for example, yellow sapphire.

Colouration
Pure corundum is colourless, often called white, and although quite rare, tends not to be valuable because it does not have much brilliance. Colours, as in many gemstones, are caused by small amounts of impurity, usually metallic oxides. This is a case where impurity is desirable.
Chromic oxide causes brilliant red colouring in corundum, thereby producing rubies.
Ferric oxide causes yellow colouration, titanium oxide produces vivid blue.
In fact the colouration of sapphire is not quite so simple as this. The titanium and iron are usually present in the form of ilmenite, a mineral which is a titanium iron oxide, TiFeO3. Ilmenite is not isomorphous with aluminium oxide. Isomorphous means being able to replace the host mineral within its crystal structure. Instead ilmenite is present as a microscopic inclusion, in the form of colloidal particles.
This colloidal nature may be responsible for other optical effects such as "silk", asterism, and colour banding.

Hardness
Corundum is very hard, having a hardness of 9 on Moh's scale, compared with 10 for diamond, and 8 for topaz. Hardness is generally a desirable feature is gemstones.
Other uses for corundum, because of its hardness, are as watch bearings, watch glasses, and as an abrasive.

Sources
Originally, the best sapphires and rubies came from Burma, where they are believed to have been mined possibly from prehistoric times. Certainly they appear to have been worked during the times of Marco Polo.
Kashmir is another source of very fine sapphires, famous for its cornflower blue stones.
Thailand, previously called Siam, is an important source of attractive sapphire.
The term Ceylon sapphire is frequently used to denote pale to medium sapphires. Unless the stone is known to originate from Sri Lanka, as it is now called, such sapphire should accurately be called "Ceylon-type" sapphire.
Currently most dark sapphires come from Australia, and the term "Australian sapphire" is often used to denote dark coloured sapphires, in a similar way to the term "Ceylon sapphire" for lighter stones.
Sapphires are also found in Montana and Colorado in the USA, India, with small quantities being found in numerous other countries.

Prices
The price range of sapphires is very large, ranging from under 1 per carat to many thousands of pounds per carat, depending primarily on colour, but also on brilliance, which is affected by clarity and cutting.

The Best and Most Valuable Colour
We are frequently informed, by partially educated customers that the darker the sapphire the better. We are equally frequently and erroneously told the opposite. If you think, even briefly, about this it becomes obvious why. A very dark sapphire would appear black, and would not be very attractive or desirable. The darkness often being caused by inclusions. An extremely pale sapphire would be colourless, and although rarer than black sapphire, is not particularly attractive or vaulable.
As usual, the truth lies between the two extremes. The most desirable sapphires are generally those with an intense blue colour, and plenty of sparkle and life. These latter two factors are usually helped by high optical clarity and skilful cutting.
Ultimately which is "best" is a subjective matter, and personal preference is important. Our usual advice to potential customers is to buy whichever colour of sapphire they personally find the most attractive. We also think it's slightly sad that we need to give this advice. Buy what you like, using your own judgment, rather than allowing yourself to be a slave to fashion and buying what you think will impress other people.

Cleaning
Sapphire jewellery can be cleaned using hot soapy water, or detergent, rinse thoroughly afterwards as detergents can cause dermatitis and allergic reactions. Enzyme cleaners should be avoided for the same reasons. Brushing with an old tooth brush to remove dirt and grease will also help. Cleaning agents containing chlorine may have a detrimental effect on low carat gold alloys, so are best avoided.

Technical Data
Chemical Composition and NameAl2O3 - Aluminium Oxide
Hardness9
Refractive Index1.759 - 1.767 to 1.770 - 1.779
Specific Gravity3.96 - 4.01
Birefringence0.009
Crystalline SystemTrigonal

Cornflower Blue

Sapphire Rings


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