Colour-change Paperweight

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


Author: prehistorian

Prehistorian & author

4 thoughts on “Colour-change Paperweight”

  1. Ha! My mum has this same paperweight, and I was just talking to dad about how it seemed to have changed colour over the years. It looks blue right through, and my mum hated blue paperweights and anyway we both remembered it being a different colour.

    I see now the florescent low energy lighting might be to blame. I shall take a look tomorrow in daylight and see if its returns to its more familiar hue.


  2. We have a caithness bon bon dish bought in the late 60s that changes from purple to greeny blue. It has never been under florescent lighting

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