Chatoyancy: Cat’s Eye Effect

Chatoyancy is an optical phenomenon in which a band of reflected light, known as a “cat’s eye,” moves just beneath the surface of a dome-shape cut gemstone.

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The name “chatoyance” originates from the French word “chatoyer,” which means “to shine like a cat’s eye.” The analogy also matches the way a
cat’s pupils will narrow to a thin slit under bright light.

In a chatoyant gemstone, the band of light will move back and forth beneath the surface of the gem as it is turned under a beam of incident light.

cat's eye collection for post

The band will also move if the position of the light is moved, or the observer moves his head to view the stone from a different angle.

The term “cat’s eye”, when used by itself as the name of a gemstone, refers to a cat’s eye chrysoberyl. It is also used as an adjective which indicates the chatoyance phenomenon in another gemstone, such as cat’s eye aquamarine.

Chrysoberyl can exhibit very sharp and distinct cat's-eyes under almost any kind of light.

Unlike other cat’s-eye gemstones Chrysoberyl can exhibit a very sharp and distinct cat’s-eyes under almost any kind of light. (Photo by: Tino Hammid)

The effect is caused by the reflection of light by parallel fibers, needles or channels in the gemstone. Usually the gemstone needs to be cut en cabochon(flat with dome-shape top) with the base parallel to the fibers for this effect to be displayed.

When the gem is rotated, the cat’s eye appears to glide over the surface. The chatoyant effect is similar to “asterism” (the star effect), except there is one straight ray instead of four or six. Occasionally a cat’s eye with two parallel rays can be seen.

In order for the stone to exibit the cat's eye effect it needs to be cut parallel to the fibers. If cut perpendicular you will end up with a dark lifeless gemstone.

In order for the stone to exibit the cat’s eye effect it needs to be cut parallel to the fibers. If cut perpendicular to the fibers (90 degrees) you will end up with a dark lifeless gemstone.

Needle-like crystals of rutile and hematite are well-known for producing a cat’s eye in many specimens.

The best cat’s-eye gems have an eye that meets the following criteria:

  • it is clearly visible
  • it symmetrically bisects the cabochon
  • it contrasts sharply with the body color of the stone
  • it moves smoothly as the stone is turned under light

If a stone features all of the above criteria and has an exceptional body color, then you have a fantastic cat’s eye gem. The importance of a desirable body color however cannot be overstated.

This amazing dome-shaped alexandrite gemstone shows not only "the cat's-eye effect" but also the "alexandrite color-change effect". (photo via GIA)

This amazing dome-shaped alexandrite gemstone shows not only “the cat’s-eye effect” but also the “alexandrite or color-change effect”. (photo via GIA)

Some gems with a highly developed chatoyancy can appear to be made of two different materials when illuminated from the proper direction with respect to the observer’s eye.

In these stones, the cat’s eye will appear to divide the stone into a zone of light-colored material on one side of the eye and dark-colored material on the other. This phenomenon is known as the “milk-and-honey” effect.

Two perfect examples of "The Milk and Honey" effect in some cat's-eye gemstones.

Two perfect examples of “The Milk and Honey” effect visible in some cat’s-eye gemstones.

Chatoyancy can be found in a number of other gem varieties. Cat’s eye tourmaline can often be found in green and pink, and larger sizes are not uncommon.

Several quartz varieties are well-known for their chatoyant effects. The most famous is so-called tiger’s eye. Others include emerald, iolite (also known as cordierite), aquamarine, andalusite, tanzanite and scapolite.

Cat’s Eye Chrysoberyl:

Chrysoberyl is considered the ‘original’ cat’s eye gem, and is in fact the only gem that can be traded as simply ‘cat’s eye’; all other varieties must be traded using very specific names, such as ‘cat’s eye quartz’ or ‘cat’s eye aquamarine.

The term "cat's eye", when used by itself as the name of a gemstone, refers to a cat's eye chrysoberyl. (photo by Tino Hammid)

The term “cat’s eye”, when used by itself as the name of a gemstone, refers to a cat’s eye chrysoberyl. (photo by Tino Hammid)

Unlike most other cat’s eyes, chrysoberyl can exhibit very sharp and distinct eyes under almost any kind of light.

The chrysoberyl species was first discovered in 1789 by renowned geologist, Abraham Gottlob Werner.

Chrysoberyl is a very rare mineral and gemstone quality deposits of cat’s eye are even rarer. Most chrysoberyl cat’s eye is sourced from Brazil, China, India, Sri Lanka and Zimbabwe.

A simple solution of warm water and mild dish-detergent is the nest way to clean your cat's-eye gemstones. Be sure to wash the soapy residue!

A simple solution of warm water and mild dish-detergent is the nest way to clean your cat’s-eye gemstones. Be sure to wash the soapy residue. Never use bleach or ammonia-based cleaners!

Chrysoberyl cat’s eye is not typically treated or enhanced in any way.

For proper maintenance of your cat’s-eye gemstones, simply wipe them using a soft cloth and plain soapy water; avoid bleach and other harsh household cleaners! Be sure to rinse well to remove all soapy residue.

We hope this article has given you a bit more of a background on the cat’s-eye effect in gemstones, but should you have any additional questions about this or any other jewellery related topics, you can always: “Ask Our Jeweller”

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This article was compiled from my personal knowledge as a gemologist, as well as numerous online sources, individuals and textbooks. If you have something to add to this article that you feel would be of benefit to others, please, do not hesitate to contact us.

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