Chrysoberyl: Cat’s Eye and Alexandrite

In my immediately previous article, I discussed rubies and sapphires and their significance to our own Sri Lankan gem and jewellery industry. 

This article discusses the next most important species of gemstones in Sri Lanka. About 10% of our gem exports amounting to about Rs 650 million are Cat’s eyes and 2% in value is Alexandrite, the most important gem varieties in the Chrysoberyl species of gems. This gemstone species commands the second place in exports, next to Sapphire, which represents about 80% of gem exports. 

Chrysoberyl has a basic chemical composition of Berylli-um Aluminum Oxide (BeAl2O4) and bears the orthorhom-bic crystal system and is only second in hardness (H=8.5) to that of Corundum (H=9). In its pure form it is colourless, but it usually contains ferric oxide and chromic oxide and these trace constituents produce different varieties, called Chrysoberyl (yellow, yellowish green, olive green and brownish green), Alexandrite and Cat’s Eye. The basic Chrysoberyl falls into the lower price range (Semi precious), while Cat’s eye is in the price range of Blue Sapphire and Alexandrite, being priced similar to that of Ruby.

Cat’s Eye 

This is a variety of Chrysoberyl with acicular mineral inclusions or tubes oriented in one particular direction of the crystal structure. In its structure, these innumerable microscopic needle-like inclusions create a silky reflection effect. These inclusions make the stones cloudy, reduce transparency and produce light reflection effects. When such stones are cut with a curved surface (called ‘en cabochon’), they display chatoyancy or the Cat’s eye effect. What is seen is a silvery streak of light, which is displayed across the curved surface. This has the striking resemblance to the pupil of a cat’s eye, hence the name. This silvery streak of light could be very sharp or diffused. And the effect could be either enhanced or diminished by the gem cutter by skilful variation of the curvature of the curved surface of the gemstone.

Gemstones like Ruby, Sapphire, and Diamond are faceted and the stones are highly priced when they are transparent and have no inclusions. But on the contrary, the Cat’s eye effect is seen well, only when the stone is not transparent but instead is either translucent or opaque. The value of a Cat’s eye gemstone will depend on the effectiveness of the ray, the size of the stone and its colour. Some common colours in which Cat’s eyes occur are gray, greenish brown, apple green, greenish yellow, brown and honey colour. Honey coloured stones are very much in demand. 

So, a Cat’s eye is not really a gemstone but a property of a gemstone as described above, because this effect is also seen in many other gems such as Tourmaline, Zircon, Enstatite, Apatite, Fibrolite, Quartz etc. In such instances the name of the variety should be used as a prefix, example Tourmaline Cat’s eye. 

When this effect is seen in Chrysoberyl it is called Cymophane in gemmology, but that name is hardly used in the trade. Instead, if the words “Cat’s eye” appear without any prefix, it always means Chrysoberyl Cat’s eye.

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