What is chatoyancy in the context of gemstones?

Chatoyancy refers to a visual effect seen in certain gemstones where a narrow band of light appears to move across the surface of the stone as it is turned. This effect resembles the slit-eye of a cat, which is where the term "chatoyancy" is derived from the French word for "cat's eye."

When a gemstone exhibits this chatoyant effect, it's typically due to the presence of parallel inclusions or fibers within the stone that reflect light in a specific way. This quality is most commonly associated with stones like tiger's eye and cat’s eye chrysoberyl.

Understanding this phenomenon is essential because it contributes to the gem's aesthetic appeal and can influence its value. Gemstones that display strong chatoyancy are often more sought after than those without it, adding to their desirability in the market.

The other options reflect different gemstone characteristics but do not accurately define chatoyancy. A glow from within relates to the concept of clarity and internal beauty, color change pertains to color-changing gemstones like alexandrite, and solid crystal formation describes the overall structure of crystals rather than specific optical phenomena. Each of these terms highlights different aspects of gemstones, yet it is the chatoyant effect that specifically defines chatoyancy.