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A bezel-set large cushion-cut stone of natural chrysoberyl alexandrite from Ceylon, $67,500 (£51,300) at Skinner.

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The stone’s most distinctive property is that it changes colour (the result of the phenomenon of metamerism) and can be green by daylight and red by incandescent light.

The first emerald mine had been opened in 1831. Other deposits were subsequently found in Brazil, India, Madagascar, Tanzania and, in the case of this large stone that provides the centrepiece to an Edwardian millegrain diamond pendant, Sri Lanka.

Alexandrite in sizes over three carats is very rare. This bezel-set large cushion-cut stone measures around 15 x 14 x 9mm. It was sold at Skinner (23% buyer’s premium) in Boston on December 3 together with a letter from the AGL stating that the stone is natural chrysoberyl alexandrite from Ceylon, with no enhancement and a strong degree of colour change. Estimated at $10,000-15,000, it took $67,500 (£51,300).