How to Explain Cat’s Eye Gemstones
Chatoyancy is the gemological name given to the curious optical effect in which a band of light is reflected in certain cabochon-cut gemstones, creating an appearance similar to light bouncing off a cat’s eye. Gem-A’s Collection Curator, Barbara Kolator FGA DGA explains chatoyancy and highlights some of the many gems in which it can occur.
Chatoyancy is the name given to the âcatâs eyeâ optical phenomenon which can be seen in certain gemstones. The term âchatoyancyâ, deriving from the French for âshining like a catâs eyeâ, denotes the effect that occurs when a bright light is shone onto a rounded, reflective surface and causes light to be reflected back in narrow line perpendicular to the observerâs line of sight. This can be seen by looking at a spool of shiny thread or some cabochon-cut gems.
What Causes Chatoyancy?
Chatoyancy occurs when a band of light is reflected from a series of thin inclusions which are parallel to each other (these may be hollow tubes or needle-like crystals of, for example, rutile or hematite).
In order to display this effect there must be enough fibers oriented in parallel to the base of the stone. The gemstone also has to be cut as a cabochon to provide the necessary rounded surface for the line of light to be seen.
As the light moves, or the observerâs eye line moves, the line appears to move too, appearing just like the way a catâs pupil will look under a bright light.
A chrysoberyl cabochon showing a sharp chatoyant effect. Image by Pat Daly, Gem-A.
The type of line produced will depend on the types of inclusions; the sharpest, finest line is produced in chrysoberyl, which is the only stone which may actually be called âcatâs eyeâ, without a qualifying species name. Such a stone will possess needle-like inclusions which form an effect known as âsilkâ. As a result, a very definite and sharp chatoyant effect is produced.
Which Gemstones Exhibit Chatoyancy?
Many species can exhibit chatoyancy including tourmaline, apatite, beryl, actinolite, demantoid garnet, scapolite, sillimanite and quartz among others but only chrysoberyl can be called simply âcatâs eyeâ, the others must be prefixed by the species name.
Tiger’s eye quartz. Image by Pat Daly, Gem-A.
Chatoyant quartz known as âtigerâs eyeâ is the only chatoyant gem produced in abundance. It can be found in South Africa and Australia and is particularly popular for use in menâs jewelry.
Cat’s Eye Gems
Catâs eyes commonly occur in yellow, yellow-brown and greenish-yellow. Some stones, such as catâs eye alexandrite, can even exhibit color change which is very rare, extremely expensive and desirable. The chatoyancy is caused by very fine needles which give a bright, sharply defined eye.
A cat’s eye alexandrite appearing green in daylight and purple-red under incandescent light. Images by Charlie Bexfield, Gem-A.
Catâs eye can also show a mesmerizing âmilk and honeyâ effect where one side of the line is milky and the other translucent. As light moves across the surface the bands part and merge like the blinking eye of a cat. This particular effect has led some to believe that the stone possesses supernatural qualities.
At 8.5 on the Mohâs scale of hardness, catâs eye is a hard and durable stone and is especially popular in Japan for menâs jewelry due to its rarity.
Catâs eye was also a very popular stone for engagement rings in Victorian times, when it was known as chrysolite. Another old name allocated to this stone is cymophane which means âwaveâ and âappearanceâ because of its optical effects.
Chatoyancy is an interesting and unusual effect worth looking out for to add interest to either a jewelry or mineral collection. Whatâs more, it doesnât have to be expensive; actinolite, sillimanite and apatite can be found quite easily, and catâs eye quartz is very popular and obtainable.
Source: Gemmological Association of Great Britain, Gem-A.com