Word Wednesday: Tenebrescence

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

Tenebrescence (ten-eh-BRESS-ens) is a reversible color change triggered by exposure to particular wavelengths of light. It is sometimes called reversible photochromism.

A mineral memory trick

Hackmanite, a variety of sodalite, is the classic example. Some specimens deepen from pale gray, cream, or pink to violet after exposure to ultraviolet light, then gradually fade toward the original color. The cycle may be repeated, although strength and speed vary by specimen.

Not every color change is tenebrescence

Fluorescence is visible while ultraviolet light is shining and stops almost immediately when the light is removed. Phosphorescence continues glowing after the source is removed. Tenebrescence changes the body color and can persist for minutes, hours, or longer before fading.

Collector tip

Use a purpose-built UV lamp safely, never look directly into it, and protect skin and eyes. Keep before-and-after photos under consistent lighting. A color response is fascinating, but it should be combined with other properties 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.