Sapphire is a Gemstone comprising of an aluminium oxide with trace amounts of Iron, Titanium, Chromium, Copper or Magnesium giving colours such as Blue, Yellow, Purple, Orange or Green.
Sapphire is one of the 3 gem varieties of a compound called Corundum (the other 2 being Ruby) and has a hardness of 9.0 on the Mohs Hardness scale.
Sapphires like other gemstones can be treated to enhance and improve their clarity and colour - and a common treatment is to heat-treat the gems - either between 500 and 1800C for several hours or in a nitrogen-deficient atmosphere for 7 days or more. This treatment rids the stone of all silk inclusions and it becomes clear under magnification.
Some examples of Sapphires are below.