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.

Candlelight, on the other hand, contains much more red light than that of other colours. The team observed that, when lit by candlelight, alexandrite emanates light that is more strongly red-dominated than that reflected by other richly coloured objects, such as fruit.

In most circumstances, the human visual system corrects for a light source’s colour cast. But alexandrite overrides that correction by thoroughly and precisely mopping up yellow and blue light without absorbing any red or green.