Sapphire

The remarkable hardness of sapphires is 9 on the Mohs scale. The cost of natural sapphires varies depending on their color, clarity, size, cut, and overall quality – as well as their geographic origin. Sapphires are found in Eastern Australia, Thailand, Sri Lanka, Myanmar, Cambodia, Madagascar. Sapphires come in different colors. Many royal engagement rings have included blue sapphires because they are more rare and unique than diamonds.

It is common practice to heat natural sapphires to enhance color and clarity. This is done by heating the sapphires in furnaces to temperatures between 500 and 1800 °C for several hours. Upon heating, the stone becomes more blue in color, and loses some of the natural inclusions called ‘silk'. The inclusions in natural stones are easily seen with a jeweler's loupe. Un-heated natural stones are somewhat rare and will often be sold accompanied by a certificate from an independent gemological laboratory attesting to "no evidence of heat treatment".

The technology of heat treatment had not been disclosed until 1982 when the heat treatment became a major issue. At that time, 95% of all the world's sapphires were being heated to enhance their natural color. But later, according to United States Federal Trade Commission guidelines, disclosure is required of any mode of enhancement that has a significant effect on the gem's value.

Blue sapphires that are eye-clean or entirely free of inclusions are uncommon, especially in larger sizes.  Blue sapphires with some internal inclusions are still highly valued, as long as the inclusions do not reduce brilliance, obscure color, or otherwise detract from the stone’s beauty.

Silk inclusions are acceptable in blue sapphires, as long as they are not so dense as to compromise color or brilliance.  In fact, silk inclusions can increase the value of a sapphire.  The heat treatments used to alter color and clarity in blue sapphires break down rutile silk, so the presence of intact silk indicates that a sapphire is unheated. 

(Adapted from Wikipedia)