Chatoyancy- A band of Light

Chatoyancy- A band of Light

I’ve always told myself that if I ever have to do a gemology trick to kids it will be showing them a cat’s eye gem. It’s a cool phenomenon and it’s just fun to see something gliding over or hovering above a stone. I’ve collected a few over the years and decided to share it here, as my first blog.

 Sillimanite Cat’s Eye

Figure 1, a Sillimanite cat’s eye is vivid against a grayish black body color. The cabochon weighs 3.2 carats, a relatively flat base for RI reading.

But first, what is it exactly?

We know inclusions usually have a negative connotation but it can also cause optical phenomenon in some gems. Chatoyancy also known as cat’s eye, is the technical term for a band of light which seems to dance on the surface of a gemstone. It happens when light is reflected from numerous inclusions like needles or hollow tubes, in parallel directions. It is best displayed when the gem is cut en cabochon with the base parallel to the inclusions. In the trade the term “cat’s eye” refers to Chrysoberyl specifically even though it can be found in several gemstone varieties like Apatite, Quartz, Kornerupine and Tourmaline.

Sillimanite Inclusion

Figure 2, shows a parallel of oriented fibers in Sillimanite that cause its chatoyancy
In Figure 3, this Sapphire displays a faint chatoyancy against a deep violetish blue body color. A closer look in figure 4, using optic illumination we can see iridescence, liquid inclusions and small crystals.

I have personally examined Sapphire roughs in a cutting workshop in Bangkok, and orienting the stone to display chatoyancy was a challenge. In the end, the dome of the cabochon had a sheen but it didn’t display chatoyancy. Sometimes, it isn’t just there, even in the hands of experienced cutters, so when it is, it’s magic.

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