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Chameleon diamonds: How heat and darkness bring out the best in these colour-changing stones

Colour origin

A .85-carat VS1 fancy grey chameleon.
A .85-carat VS1 fancy grey chameleon.

Chameleon diamonds are classified as having natural colour, as their behaviour cannot be duplicated and there is no known treatment to cause the chameleon effect in other stones. This phenomenon is not a ‘colour shift’ that can be observed when a yellow diamond with strong green-yellow fluorescence appears more green under fluorescent light (and daylight with ultraviolet [UV]). Many treated green-yellow diamonds (i.e. neon) colour-shift from more yellow to more green when the colour centre (caused by irradiation and HPHT treatment) in the green part of the visible spectra (503nm) is triggered by fluorescent light.

Shapes and sizes

As mentioned, chameleon diamonds comprise a small percentage of the coloured diamond group. The larger the chameleon diamond, the more dramatic the colour change. Similar to most coloured diamonds, chameleons are usually cut into fancy shapes like princess, emerald, oval, radiant, marquise, and pear, rather than round brilliant.

Reaction to UV radiation

Based on the findings of our testing, there are basically two types of responses to UV radiation:

  1. Classic group: strong chalky-yellow to chalky-orangey-yellow under long-wave and one degree weaker under short-wave UV.
  2. Reverse group: strong chalky-blue to blue under long-wave, and weaker chalky-blue to yellow under short-wave UV.

 

Spectroscopy

Hydrogen was identified in all samples by Fourier transform infrared spectroscopy (FTIR) (3107cm-1 peak). We determined most chameleons are Type IaA diamonds. Some minor nickel-related emissions were detected by photoluminescence (PL) spectroscopy in most of the samples.

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