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A variety of sapphire that has a silky structure and when cut en cabochon shows a 6-rayed (sometimes 12-rayed) star in reflected light, due to the phenomenon known as asterism. The stone must be precisely cut, aligned with the vertical axis, as otherwise the result will be an off-centre, crooked or dim star, or even the absence of a star.
Such star stones are natural or a synthetic gemstone. The largest known star sapphire, grey-blue and weighing 12,6 kg (about 63000 carats), was found in 1966 at Mogok in Burma.
From: An Illustrated Dictionary of Jewelry, autor: Harold Newman, publishers: Thames and Hudson