What is pleochroism?

• A. The phenomenon of showing identical colors from all directions.
• B. A surface phenomenon that does not affect color.
• C. The phenomenon of showing different colors when viewed from different crystallographic directions.
• D. Color changes due to daylight exposure.

Answer: (C)

Pleochroism is the phenomenon where a gemstone shows different colors when viewed from different crystallographic directions. This happens because anisotropic crystals absorb light differently along their various optical axes, so the color you see changes as you look along different directions or rotate the stone. In practice, you may notice two colors (dichroism) or even three colors (trichroism) in stones like tourmaline or sapphire under polarized light or with a dichroscope. This is a bulk property of the crystal’s internal structure, not a surface effect, and it isn’t caused by daylight exposure or environmental factors that produce metamerism or photochromism.