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Gemstone Phenomena: Chatoyancy

| 2 min read

We’ve previously told you about the phenomena known as asterism. Chatoyancy is caused by a similar process and is just as extraordinary, but looks a little different. Named after the French 'oeil de chat', meaning 'cat’s eye', chatoyant gems feature a single band of light which magically appears to open and close as the gem is turned in the light. Many gemstones are capable of this effect to some degree, but very few can give a very strong and obvious cat’s eye effect.

Chatoyancy is caused by similar forces as asterism. Light reflects off very thin parallel inclusions in a gem, known as its ‘silk’, creating a band of light across its surface. These inclusions can be hollow tubes, crystals or other long structures aligned in one direction. The cat’s eye effect runs at right angles to these, seeming to dance under the surface of the stone. The best analogy is picturing a spool of silk thread and the light reflecting in a band at the surface, at 90° to the threads.

Cat's Eye Chrysoberyl

The gem cutter must carefully choose the right orientation to cut the rough and will cut the gem en cabochon in order to show the effect at its best. Often, the underside of the gem will be left unpolished, to trap the light within the gem and stop it escaping out of the bottom.

Cat's Eye Chrysoberyl gem

We bring you a few different gemstones that show this effect. Cats Eye Tourmaline and Cats Eye Alexandrite are two of the most popular. Alexandrite with this effect sometimes will display a slightly different colour or opacity on either side of the line of chatoyancy, a phenomenon called 'milk and honey'. Rutilated Cats Eye Quartz shows the ‘silk spool’ analogy very well. Rutiles run through the Quartz in a curve. The cabochon follows this shape and the light catches the top of the curves, giving a wonderfully unique effect.

TIGER’S EYE

Tiger's Eye ring

One of the most famous chatoyant gemstones is Tiger’s Eye. This is a gem from the Quartz family, with stunning golden or red tones. Intergrowth with other minerals and layers of impurities causes the chatoyant effect to be seen across many different surfaces and in many directions. For this reason, Tiger’s Eye can be faceted in different ways and will still show the effect.

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PIETERSITE

Pietersite ring

A rare and fascinating gem, Pietersite was discovered in Namibia in 1962 by mineral dealer Sid Pieters. It is an aggregate, meaning that it is a gem made up of fragments in a matrix. This matrix includes Tiger’s Eye, which gives it a distinctive chatoyancy. Unusually, the chatoyancy is in a swirling or streaked pattern, most often in blue and gold hues. With its extraordinary ‘storm-cloud’ appearance, Pietersite is a beautiful and rare collector’s gem for any connoisseur.

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MORE GEMSTONE PHENOMENA

Asterism Aventurescence Fluorescence and Phosphorescence Polymorphism Labradorescence Tenebrescence Twinning