Pleochroism and chatoyancy are two ways that light is refracted or reflected in gemstones, (and some wood).
Pleochroism is an optical phenomenon in mineral grains of rock that appear to be different colors when viewed at different angles.
This is caused by a double refraction of light.
Some gemstones that are pleochromic are Adalucite,Iolite,Tanzanite,and Zoisite.
Chatoyancy is the optical reflection seen in some gemstones. Caused by the fiberous inclusions or cavities in the rock. they are always perpendicular to the direction of the fiber.
Chatoyant stones are Tiger Eye, some Quartz,(Iris rare!) Chryoberyl, Beryl (Aquamarine, Tourmaline and Moonstone).