chatoyancy, asterism, gems

Gübelin explains the optical principles responsible for chatoyancy (the cat’s-eye effect) and asterism (the star effect) in gemstones. He relates these phenomena to the reflection and scattering of light by numerous regularly oriented inclusions within a transparent or translucent host crystal. Parallel inclusions produce a single luminous band perpendicular to their orientation, giving the cat’s-eye effect when the stone is appropriately cut en cabochon. When inclusions occur in two or more intersecting crystallographic directions, the corresponding bands intersect to produce a star. The geometry and orientation of the inclusions, together with the orientation and shape of the cabochon, therefore determine the position and appearance of the optical effect.