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School of Physics and Astronomy

CCMMP Research Highlights in Nature - part 2

How a chameleon gemstone changes from red to green

The rare jewel alexandrite outwits the human eye’s colour-correction system.

The gemstone alexandrite has the remarkable ability to change colour under different lighting. Now, scientists have found that this trick is an optical illusion that hinges on how humans perceive colour.

Alexandrite stones appear to be a brilliant emerald green in daylight, but a rich ruby red under candlelight. By measuring the light that the stones transmit, David Dunstan at Queen Mary University of London and his colleagues found that alexandrite’s chromium atoms absorb both yellow and blue light, leaving green and red light to reach a viewer’s eye. That helps to explain the gem’s green tones when illuminated by sunshine, which is dominated by green wavelengths.


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