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The Learning Kingdom's Cool Fact of the Day for July 30, 1999
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What makes the star in a star sapphire?
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A star sapphire is a gem that shows a six-pointed (or rarely,
twelve-pointed) star-shaped image when viewed under a point light
source. There are also other star-reflecting minerals, all of which
show stars for the same reason: there are thousands of tiny needle-
shaped crystals within the gem.
Star sapphires contain microscopically thin needles of the mineral
rutile. The crystals are oriented along the molecular axes of the
sapphire crystal, which has six-way symmetry. Light is reflected by
the shiny rutile crystals into the six-pointed star.
Sapphires are crystals of aluminum oxide, also known as corundum.
Corundum is the second hardest mineral, after diamond. Rubies are
also corundum crystals.
More about sapphires and rubies:
http://www.minerals.net/gemstone/gemstone/sapphire/sapphire.htm
Another Cool Fact about sapphires:
http://www.cool-fact.com/archive/1997/11/17.html
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