Alexandrite is an extremely rare gemstone with the property to change from purple to green. It appears purple under artificial light and green when viewed in daylight. The gemstone is named for Tsar Alexander II of Russia, on whose 16th birthday it was discovered, and fine examples can cost up to $100,000 per carat. I was quite astonished to discover that a rather less expensive Caithness glass paperweight in my possession for the last twenty years can do more or less the same thing! In daylight, or under normal incandescent or halogen lighting it appears a rich purple:
But I recently installed compact flourescent lighting, and under this the paperweight turns blue:
The likely explanation for this phenomenon is that the purple-coloured glass contains a mixture of red and blue colourants. Under daylight or a continuous-spectrum artificial light, these appear purple. However flourescent lighting is deficiant in red wavelengths and when illuminated by such lighting, the red colourants are not seen, causing the paperweight to appear blue.
© Christopher Seddon 2008