Corundum is a precious gemstone. It gives two beautiful varieties of precious stones which are sapphires and rubies.
Star phenomena are often observed in rubies and sapphires due to their asterism or internal reflection. The cabochon form elevates the worth of low quality Corundum. Star sapphire is chemically aluminium oxide and is formed as trigonal crystals. A glimpse into a sapphire crystal shows its trigonal symmetry. It consists of a colourless silky centre surrounded by opaque blue outgrowths.
Despite their beauty, the mechanism of internal reflection is quite straightforward. Once the sapphires or rubies cool down from a semi liquid state, titanium dioxide present in them crystalizes and the structure becomes solidified.
Following the formation of structure building in the sapphire, rutile needles grow along crystal structure. This results in the formation of microscopic needles at 60 degree intervals. However, the star only becomes visible if rutiles grow sufficiently densely in six different directions. The external light which reflects perpendicular to the rutile growth is responsible for the occurrence of star effect.
The cutting in star sapphires plays a crucial part. Gem cutter must be aware of precautions in order to get a well defined and properly oriented star. A sufficient and care is needed for proper orientation during cutting star rubies and sapphires.
A misshapen bi-pyramidal star sapphire obtained from the Kyauk-pyat-that, Mogok, is oriented properly for cutting it into star stones.
Moreover, if you see any perfect star sapphire, be careful. There are also a lot of man made, painted star sapphires in the market. Always judge before buying. Generally star sapphires are not heated . Mostly come from natural realm.
The popular star sapphire is " Star of Asia" . It was 330 carats found in Srilanka and it was an outstanding gemstone.
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