Tanzanite is one of the gems that show which phenomenon?

• A. Asterism
• B. Chatoyancy
• C. Adularescence
• D. Pleochroism

Answer: (D)

Pleochroism is the ability of a gem to show different colors when viewed from different directions because the mineral absorbs light differently along its crystal axes. Tanzanite, a blue-violet variety of zoisite, is strongly pleochroic. Depending on the orientation, you can see blue in one direction and violet in another, and in some crystals a third color such as burgundy. This property is what makes tanzanite particularly striking, and cutting often aims to maximize the pleasing color by selecting the orientation that displays the desired hue.

Asterism, the star effect, comes from needle-like inclusions arranged to form a star; chatoyancy, the cat’s-eye effect, arises from parallel fibers or needles; adularescence is the soft, milky bluish glow seen in moonstone. These different phenomena are not what tanzanite exhibits—its hallmark is pleochroism, the color change with viewing angle.