Alexandrite – colour change capabilities
A rare variety of chrysoberyl, the alexandrite is prized for its ability to change colour. In daylight, when light waves are shorter it looks bright green whilst in the long waves of produced by candles or light bulbs it takes on a red-to-brownish appearance.
The colour change is due to the presence of chromic oxide and is generally more dramatic than that found in other types of stones that can have pleochrmic properties, such as sapphires, tourmaline and apatite.
As with the chrysoberyl, its hardness is 8.5, making it one of the hardest as well as the rarest stones. Until 1987 the only source was the Ural Mountains in Russia. And in the 150 years since its discovery, and subsequent naming after Alexander II, supplies had been largely depleted. However, in the late 1980’s new deposits were discovered, in Brazil and shortly after that on the border of Tanzania and Mozambique.
With a high refractive index of 1.745-1.757 there is plenty of fire to be had in stones that are cut either in cushion or brilliant designs which tend to the most common for the Alexandrite.
Specimens that exhibit a strong colour change, from emerald green to raspberry, are very rare in sizes greater than 5ct and reach prices in excess of USD100,000 per carat.