Chrysoberyl: Cat's Eye and Alexandrite

Cats eye and Alexandrite are the most important gem varieties in the Chrysoberyl species of gems.

Has a basic chemical composition of Beryllium Aluminum Oxide (BeAl2O4) and bears the orthorhombic crystal system and is only second in hardness (H=8.5) to that of Corundum (H=9). In its pure from it it 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 Bat’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 prices 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 structures. 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 ‘encarbochon’), 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 striking resemblance to the pupil of a cat’s eye. hence the name. This silvery streak of light could be very sharp of diffused. And the effect could be either enhanced of diminished by the gem cutter by skillful 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. 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 gens such as Tourmaline, Zircon, Enstatite, Apatite, Fibrolite, Quartz etc. In such instances the name of the variety should be used as a prefix, Tourmaline Cat’s eye.

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