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

By Branko Deljanin

At 31.32 carats, this oval stone is the world's largest chameleon diamond.
At 31.32 carats, this oval stone is the world’s largest chameleon diamond.

Chameleon diamonds have an unusual ability to change colour temporarily, either when gently heated to approximately 150 C (thermochromism) or after prolonged storage in the dark (photochroism). The stable colour shown by chameleon diamonds is typically greyish-yellowish-green to greyish-greenish-yellow (i.e. olive), whereas the unstable hue is generally a more intense brownish or orangey-yellow to yellow. After heating, a chameleon diamond quickly returns to its original colour. The change in colour after storage in the dark is usually not as dramatic as that seen with heating, and for some members of the trade, a temporary photochromic colour change must be present for a diamond to be referred to as ‘chameleon.’

When these rare stones are certified by a reputable lab, they generally command a premium over coloured diamonds exhibiting the same colour, but without the ‘chameleon effect.’

According to some sources, diamond broker Peter K. Kaplan is generally considered to be the first person to note the wonder of chameleon diamonds when in 1943, he observed a stone change colour after it was placed on a very hot polishing wheel. The diamond’s owner had also noticed the light yellow-green stone turn dark green after storing it in a jewellery box. Not knowing what she had was a highly valuable diamond likely worth a lot more than the relatively small amount she paid, she sought to return it, believing its ability to change colour was a defect. Over the years, chameleon diamonds have been the subject of a great deal of study (including research by this article’s author and an international team of gemmologists) to determine how they change colour.*

Chameleon diamonds are a truly exclusive gift of nature. The largest found to date weighs 31.32 carats (Figure 1, page 2), although one of the most beautiful chameleon diamonds is a stunning 8.04-carat radiant-cut stone graded by Gemological Institute of America (GIA) as a fancy dark grey-green chameleon. Due to colour rarity and size, this diamond is valued at $2.1 million US. And back in 2011, Christie’s Hong Kong auctioned off an 8.80-carat chameleon diamond ring for $590,000 US.

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