Tanzanite is one of the gems that show which phenomenon?

• A. Iridescence
• B. Metamerism
• C. Chatoyancy
• D. Pleochroism

Answer: (D)

Pleochroism is the phenomenon being tested. It describes how a gemstone can show different colors when viewed from different crystallographic directions, because light travels through the crystal with different absorptions along its axes. Tanzanite, a blue-violet variety of zoisite, is notably strongly pleochroic (often described as three-colored): you can see blue in one orientation, violet in a perpendicular orientation, and a brownish/yellow tone from another direction. This property is a key identifying feature and also guides how cutters orient the gem to showcase the desirable color. The other listed effects describe different color behaviors—iridescence comes from interference effects on microstructures, metamerism from colors changing under different lighting, and chatoyancy from a cat’s-eye effect due to fibrous inclusions; these are not the hallmark of tanzanite.