Despite having similar names, chrysoberyl is a completely different mineral to beryl, the group of gemstones to which emerald and aquamarine belong. Chrysoberyl is a very hard stone and is rated 8.5 on the Mohs scale of hardness sitting just below ruby and sapphire. In this article we will explore some of its other characteristics as well as have a look at the rare and valuable variety of chrysoberyl known as alexandrite.
Chrysoberyl can be yellowish-green, green and brown. It has a bright, vitreous (or glassy) lustre whilst displaying low dispersion of light. Some chrysoberyls can exhibit chatoyancy, or, the cat’s-eye effect. Indeed, when the term cat’s eye is used on its own without further qualification with regards to the type of stone it is being used to refer to, one can assume that we mean cat’s eye chrysoberyl. For an explanation of the cat’s eye effect, please see my blog Star Rubies and Sapphires. Dark brown chrysoberyls show strong pleochroism in the form of different shades of the same colour. For an explanation of pleochroism, see my blog Tourmaline.
A rare variety of chrysoberyl, alexandrite appears as different colours depending on the light in which it is viewed. It is so called because the first stones described were from the Ural mountains in Russia and were named in honour of the future Tsar Alexander II. At its best it is a strong raspberry red in incandescent light, an even stronger red in candlelight and a pleasant bright green in daylight. It is strongly pleochroic showing red, orange and green in daylight. An even rarer variety displays chatoyancy.
Synthetic chrysoberyl can be produced by using the flux melt method as described in my blog Synthetic Emerald. Because synthetic and natural alexandrites have the same constants when it comes to gem testing, so the observation of inclusions becomes vital in their detection. We are looking for flux filled cavities, healed fractures, platinum platelets from the platinum crucible in which the whole mixture was originally heated and straight growth lines, amongst others.
If you have any chrysoberyl that you would like to sell, please contact Simon Rufus, gemmologist at Grand Auctions, Folkestone, Kent for further details.