Though its fame is shared with its “Big Three” counterparts, ruby and emerald, sapphire has long been one of the world’s most beloved gemstones.
The deep Blue Sapphire or Neelam gemstone, as it is popularly known, comprises Aluminium Oxide (Al2O3).
While the Diamond is considered the hardest among all gemstones, the Blue Sapphire or Neelam stone comes a close second when we speak about the hardness quotient. The hardness of this gemstone on the Mohs scale is 9.
The specific gravity of the Blue Sapphire/Neelam gemstone hovers between 3.99 and 4.00, and its refractive index varies between 1.760-1.768 and 1.770-1.779.
A large number of famous sapphires have been listed in history. For example, the well-known American gem dealer Harry Winston possesses a 337.10-carat gorgeous sapphire.
A 536-carat star sapphire known as the Star of India, probably the most prominent such gem in the world, and a minor gem of 116cts called the Midnight Star reside at the American Museum of Natural History in New York City.
While the National Museum of Natural History, Washington D.C., houses the 422.99-carat Logan sapphire and the 182-carat Star of Bombay.
Another example of a huge-sized sapphire is the 2302-carat sapphire that Norman Maness carved into the form of Abraham Lincoln’s head.
Due to the rare nature of the large sapphire gems, gems above 15 carats with good color and clarity grade are considered highly exotic.
Fine medium-dark blue sapphires of rich tone come from Kashmir in India. With their cornflower blue color and slightly milky appearance, Kashmir sapphires are considered the most beautiful gemstones.
Color zoning (the uneven distribution of coloring in a gemstone) in a blue sapphire of natural origin is a widespread phenomenon.
Thus, a rough sapphire stone must be cut and faceted well-planned to obtain a beautifully colored finished gem.
Sapphires get their beautiful color tone from the trace elements iron and titanium.
A Natural Blue sapphire is commonly cut into an oval shape to retain maximum weight and obtain a high-carat polished sapphire gem.
But other shapes like round, cushion, octagonal, and rectangles are also not uncommon and depend on the native cut provided.
Natural Blue Star Sapphires have to be cut in cabochons to display the asterism in the gemstone.
Color zoning and silk (included in rutile needles) are the most striking features of the sapphire gem. Other visible inclusions that are commonly found are crystals and feathers.
Sapphire chemistry & classification
ChemicalName: aluminum oxide
chemical formula: Al2O3