The Star of Asia One of the most beautiful asteriated sapphires in the world, weighing in at a whopp
The Star of AsiaOne of the most beautiful asteriated sapphires in the world, weighing in at a whopping 330 carats, it has excellent deep blue colour and a very sharply defined star (the main criteria of value in these stones). The optical effect comes from the light reflecting off needles of rutile, a titanium dioxide mineral that we covered at http://on.fb.me/1EXvics. As the corundum cools below a certain point, the atoms that had coexisted in a flux separate and form distinctive minerals, a process called exsolution. The aluminium oxide turns into the sapphire, and the needles of rutile (known as silk in the gem trade) form along three lines of direction that reflect the crystal structure of the sapph, and are clearly visible in the photo.To reveal the effect stones have to be cut as cabochons, domed stones usually with a flat base for setting, and exposed to a bright light source. This stone has a history before it entered the Smithsonian gem collection in 1961, having belonged during the Raj era to the Maharaja of Jodhpur. Unfortunately for them, at independence and in the decades after, the pensions the Indian kings and princes had been promised when agreeing to join their princedoms to the new Indian republic never materialised, and many stunning gem collections were dispersed into the gem trade in order to put bread on the table.The rough came from the ancient mines near Mogok in Burma, still a source today for some of the world’s finest stones. Unlike most sapphires in the trade, star stones are not roasted in ovens very often, as one of the main reasons for doing this is to remove the rutile, improving the clarity and sometimes changing the colour, which would of course destroy the star. In ancient days, such stones were seen as protective talismans for travellers.We did another post on a famous asteriated stone, the Rosser Reeves star ruby, also in the National gem Collection at http://tinyurl.com/lx88px4LozImage credit: Chip Clarkhttp://geogallery.si.edu/index.php/en/1001393/star-of-asiahttp://www.gemselect.com/gem-info/star-sapphire/star-sapphire-info.php -- source link
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