Cat’s Eye Gemstones: Chatoyancy

Fine color Cymophane with a sharp and centered...
Fine color Cymophane with a sharp and centered eye. (Photo credit: Wikipedia)

Most folks have seen the tiger’s eye type of cat’s eye and the similar hawk’s eye. This is a silky orange (or blue for hawk’s eye) sheen to a usually opaque stone. More valuable and exotic is a cat’s effect in a translucent stone. There are many gemstones that can exhibit the cat’s eye effect. However, in the gem trade, a cat’s eye stone means cymophane, which is a variety of chrysoberyl. (Another variety of chrysoberyl is the famous color-changing alexandrite. The transparent variety without a cat’s effect is just called chrysoberyl.)

Chatoyancy is the fancy term for “cat’s eye” and literally comes from the French chat (cat) + oeil  (eye) or variously the present participle of chatoyer. The effect is generally caused by very fine crystals within the gem arranged in parallel. The easiest way to visual how this works is to move a spool of shiny thread under a single point light, such as the sun or a bright bulb. The silky sheen on the threads is essentially the same sheen you are seeing off the microscopic, aligned inclusions in a chatoyant gemstone.

English: A cabochon of tiger's eye
English: A cabochon of tiger’s eye (Photo credit: Wikipedia)

In general, for a good cat’s eye effect, you need enough inclusions to make the stone translucent although a faint cat’s eye may occur in mostly transparent material. The inclusion is often rutile, a form of titanium oxide which can also form larger, beautiful golden inclusions in quartz, i.e., rutilated quartz. If the inclusions are oriented on a multi-fold axis, you can get asterism but that’s a subject for another blog post. Interestingly, rutile inclusions can be the cause of a bit of cloudiness to otherwise fine rubies or sapphires. Often heat treatment will cause the rutile to dissolve back into the gemstone, clarifying it and greatly improving its value.

Gem.rutile.600pix
Polished rutilated quartz (Photo credit: Wikipedia)

In hawk’s eye, quartz replaces croidolite fibers (a type of asbestos). In tiger’s eye, the iron in the croidolite has oxidized to a brownish color. In thecommon and quite inexpensive variety, the stones are opaque. However, there is some hawk’s eye and tiger’s eye that is more transparent. In most cat’s gemstones, the stones are translucent, often with a beautiful glow as in the picture above. A very fine cat’s eye is said to “open up” as it is turned in the light. That is, the band is general very sharp but can widen a bit as the stone is turned.

Cymophane is highly prized and can get somewhat expensive, especially above a carat. In addition to body color, the sharpness and centering of the eye effect will determine value. Cat’s eye effects in other gemstones can vary in price depending, as always, on rarity and beauty. In the last few decades, you can also find artificial fiber optic crystals polished as spheres, bears or cabochons. These are very affordable and often are dyed in vibrant colors.

For the fantasy writer, cat’s eyes gems offer another dimension to your ornaments, especially if you describe the way the eye moves across the gem. Maybe in your fantasy novel, when the eye “opens” you actually see a pupil or there is some other magic effect, say a flash of light that illuminates secret letters. And “cymophane” is a word that might add a touch of the exotic to your project.

PS: draft 2 is complete on my current project 🙂 Next up is an outline (ugh) then a ‘quick’ draft 3 before letting readers take a look at it.

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