Sapphire is any gemstone-quality corundum. (The red variety of corundum is also known as ruby.)
When color is not specified, sapphire refers to the blue variety. Pink, yellow, green, white, and parti-color (multi-colored) sapphires are often valued less than the blue variety of the same quality and size. However a pink-orange sapphire, called a padparadsha, is highly prized. They were found in many countries especially in Asia such as India, Sri Lanka, Thailand, Myanmar, and Cambodia.
It is the impurities in the aluminium oxide crystal that give the color variations, with different impurity chemical elements giving the different colors that can be found. Pure sapphire is transparent. Traces of iron and titanium give sapphires a blue color. The crystals are exceptionally hard, with only diamond being harder among natural gems. They have a hardness of 9 on the Mohs hardness scale (Diamond is 10).
Gem quality sapphires and rubies occur naturaly and can be easily and cheaply produced in the laboratory. The chemical compositions and physical properties are identical to the natural sapphires. The tell-tale sign of synthetic sapphires is the crystalline growth lines which are usually curved due to the pulling during the accelerated crystal growth process.
A version which shows an asterism is called a "star sapphire" (see picture above). Although natural sapphires can show an asterism, the shape of the star is usually somewhat irregular and sometimes indistinct. A manufactured star sapphire called the Linde Star shows a very regularly-shaped and distinct asterism because the formation process is more tightly controlled than the natural version.
The Logan sapphire is one of the largest blue sapphire gems known. It weighs 423 carats (84.6 g).
Lady Diana Spencer's engagement ring from Charles, Prince of Wales was a sapphire ring.
Cornflower blue is one of the most popular colors for sapphires (the other choice color a deep royal blue), though there is little objective consensus about which shade of blue is the most cornflower or the most desirable.
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