Word Wednesday: Pleochroism

Welcome to Word Wednesday, Wolfstone Rock Shop’s weekly look at the language of rocks, minerals, and gems.

Pleochroism (plee-OH-kroh-iz-um) is the ability of certain colored crystals to show different colors or color strengths when viewed along different crystallographic directions.

Why does the color change?

In anisotropic crystals, light traveling in different directions can be absorbed differently. Rotate the crystal and one direction may appear blue while another appears green, brown, violet, or nearly colorless. This is a directional optical property, not the same as a surface rainbow or a color change caused only by switching light sources.

Examples

Iolite may show blue-violet, gray, and yellowish tones. Andalusite, tourmaline, tanzanite, and kunzite can also show noticeable pleochroism. Gem cutters consider the effect when orienting a stone because direction influences the finished color.

Collector tip

Use neutral daylight and rotate a transparent specimen slowly against a pale background. Some effects are subtle and are easier to see with a dichroscope. Do not use pleochroism alone for identification.

See this texture and other mineral features in person at Wolfstone Rock Shop, 1857 N Casaloma Dr, Appleton, WI 54913. Inventory changes often, so call (920) 809-8577 if you are looking for a particular example.