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When a gem contains a series of parallel inclusions, homogeneously distributed in its volume, light reflection from them can produce a moving luminous band, with resemblance to a cat’s eye. This effect, known as chatoyancy, generates very original looking gems, coveted by gem collectors and jewelry designers.
The classical “cat’s eye” corresponds to the chrysoberyl species. It has greenish yellow color that makes it even more similar to feline eyes. Chatoyancy is traditionally attributed to tiny needles of exsolution rutile, although other mineral species can be also involved. These inclusions are so thin that they are not distinguishable to the naked eye and even through 10x loupe.
Cat’s eye chrysoberyl from Sri Lanka, 10.84 ct, mounted in gold with diamonds. Courtesy of Jeffery Bergman, Primagem.com
The same effect can be caused in many other gems by solid inclusions of parallel needles that act in the same manner, generating a moving reflection band. Such inclusions can be formed by an exsolution process or simply trapped as aggregates of parallel needles, quite common in nature. Classic examples of cat’s eye effect caused by solid inclusions are chatoyant varieties of quartz, such as tiger’s eye, bull’s eye and hawk’s eye, with solid fibrous inclusions of amphiboles.
Chatoyancy in quartz from Brazil, caused by crocidolite inclusions. Field of view 17 mm.
Cat’s eye effect caused by parallel aggregate of rutile needles distributed in all the volume of transparent quartz.
Field of view 15mm.
Parallel growth channels can also constitute a cause for chatoyancy effect in gems like beryl, tourmaline and apatite, among others. Growth channels are usually larger than thin needles in chrysoberyl and can be visible even by naked eye, providing an easy way to distinguish cat’s eye chrysoberyl and apatite that can have very similar color.
Brazilian green tourmaline with chatoyancy effect. Field of view 20 mm.
Apatite from Kenya, with color and chatoyancy effect similar to chrysoberyl cat’s eye. Field of view mm. length 15 mm.
There is also a glass imitation of chatoyant gems made from a colored fiber optic to provoke a similar visual effect. These glasses come in a variety of different colors and are easily distinguishable from natural gems by observation from the side view of the cabochons, in direction parallel to glass fibers.
Different colors of glass imitations of chatoyant gems (left), and side view of one of the beads under magnification (right, field of view 3.8 mm).
Chatoyancy is better seen under a punctual light source like halogen light bulbs or a light torch. Illumination coming from a wide source, such as natural daylight or fluorescent tubes, will reduce its visibility.
Cat’s eye gems are always cut in cabochons. Correct rough orientation is very important to reveal the chatoyancy effect properly, with a light band moving in the complete area of the gem, parallel to its length and centered in the middle of the gem in front view. For that, the base of cabochon should be located parallel to chatoyancy causing inclusions, and the length of the cabochon should be perpendicular to their direction.