Thermochromic and photochromic behaviour of “chameleon” diamonds

Chameleon diamonds typically change color from greyish-green to yellow when they are heated or cooled (thermochromic behaviour) or kept in the dark (photochromic behaviour). Those diamonds are type Ia with A aggregates largely dominating the nitrogen speciation. They contain moderate to high amounts of hydrogen, in addition to some isolated nitrogen and traces of nickel. Their UV Visible spectrum comprises the continuum typical of type Ib material, the 480 nm band and a broad band centred around 800 nm. It is mainly modified in the red part of the visible spectrum when the color change occurs. We here propose an electronic model consistent with all observed color behaviours, based on the premise that yellow is the stable color whereas green is the metastable one. The chameleon effect is possibly linked to a nitrogen–hydrogen complex.