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Corundum (var. star sapphire)

Photograph of a double star sapphire (NMNH G9219) from the National Gem Collection
Photo by Chip Clark. Click to zoom.
Catalog Number NMNH G9219-00
Locality Sri Lanka
Weight 70.79 ct

Gift of Mrs. G. B. Pearson Jr. in 1982.

This platinum ring has a round cabochon star sapphire as the centerpiece, accented with round brilliant cut diamonds set in “leaf-like” prongs, and baguette cut diamonds fanned on the sides of the shank. Star sapphire is considered a phenomenal stone. It is an interesting member of the gem kingdom because it displays an optical phenomenon called asterism. Asterism is caused by light reflecting off of parallel bundles of tiny hollow tubes or fibrous crystals of another mineral inside the gemstone. When a stone is cut with a domed top and flat bottom, called a cabochon, the reflected light is focused into bright bands that form the “star.” Needlelike crystals of the mineral rutile inside the sapphire crystal that are aligned in three directions, intersecting at120°, form a 6-rayed star. This large star sapphire from Sri Lanka exhibits a rare 12-rayed star, probably caused by two sets of mineral inclusions oriented at 30° from each other. The gem is a very dark blue almost black color and weighs 70.79 carats. The diamonds accenting the ring have a total weight of 3.37 carats.


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