What is Chatoyance in Rocks and Minerals?
Chatoyance is an optical effect in which a narrow band of light appears to float beneath the surface of a stone and move as the stone or the light source is turned. It's caused by light reflecting off fine parallel fibres or structures inside the stone, and is best known in tigers eye and cats eye chrysoberyl.
Chatoyant, Chatoyance, and Chatoyancy
Chatoyance is an optical phenomenon that can be seen in certain rocks, minerals, and gemstones. The words chatoyant, chatoyance, and chatoyancy are all used to describe the same optical effect.
Chatoyant is the adjective, so a stone is described as being chatoyant. Chatoyance and chatoyancy are both nouns, and either is correct when describing the effect itself.
The word originates from the French verb 'chatoyer', meaning to shimmer or shine like a cat's eye.
What Causes Chatoyance?
Chatoyance is caused when light reflects off fibrous inclusions or parallel internal structures within a stone.
It produces a narrow band of reflected light from beneath the stone's surface. As the direction of light changes, this band moves.
In stones like tigers eye, chatoyance gives the impression of movement on the surface of the stone.
The minerals best known for being chatoyant are tigers eye and chrysoberyl (cats eye), but it can also be seen in several other minerals.
Why Chatoyant Stones are Cut as Cabochons
Chatoyance is best seen in stones cut as cabochons, as the curved surface allows light to interact with aligned internal fibres, producing a single, moving band of light rather than dispersed reflections.
The strength of the effect also depends on the cutter's skill, because the fibrous structures responsible for chatoyance must be aligned parallel to the surface of the stone.
Capturing chatoyance in a photo is extremely difficult because it's totally dependent on the angle of light and the angle from which the stone is viewed. It's most visible when either the stone or the light source moves.
Article Picture
The stones in our picture are elbaite, which is a variety of tourmaline. Photo: Courtesy of Stan Celestian.
