The Rosser Reeves star ruby. Reputed one of the finest in the world, this 138.7 carat beauty graces

The Rosser Reeves star ruby.Reputed one of the finest in the world, this 138.7 carat beauty graces the collection of the Smithsonian museum of natural history. Its quality is exceptional in all three main factors, colour, transparency and sharpness of asterism (as the star phenomenon is called). It was donated to the museum in 1965 by an advertising mogul, after whom it was named. When he bought it, the stone was scratched and therefore slightly recut from 140 carats, better centering the star in the process. The star is caused by needles of a titanium oxide mineral called rutile. As the corundum was forming, absorbing all the aluminium and oxygen in the mother ichor, titanium was concentrated in the remaining solution and exsolved into very fine needles spread throughout the stone. Because ruby crystallises in the same system as quartz, it produces hexagonal crystals.In a similar way the needles exsolve along the three internal axes of the crystal (the fourth is depth), forming three sets of lines intersecting at 120 degrees. In order to reveal the star, the stone must be cut into a domed cabochon like this one, and the needles then reflect light back out of the stone, revealing the crystalline pattern of the mineral as a sharp star. 12 rayed and double stars also exist. As a bonus the needles can tell geologists about the conditions of the metamorphism in which the stone grew. Garnet crystallises in a different system, and therefore produces four rayed stars.The rough came from Sri Lanka. The characteristic inclusion for corundums from this locality is long fine needles of rutile throughout the entire stone. Their thinness contribute to the stone’s quality, as coarser needles both diminish transparency and make for a less sharp star.Lozhttp://geogallery.si.edu/index.php/en/1001784/rosser-reeves-star-rubyImage credit: Chip Clark -- source link
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