Scientists Say: Tenebrescence

Tenebrescence (noun, “Ten-eh-BREH-sənse”)

Tenebrescence is a property of some minerals that change color under certain lights, such as ultraviolet (UV) or bright sunlight.

These color shifts last even after light exposure ends. The minerals do eventually fade back to their original color. But that takes time. Some fade in a matter of seconds. Others take hours or more.

This slow fade sets tenebrescence apart from fluorescence. That’s another light-related mineral property. Fluorescent minerals also color shift. But the effect ends when the light exposure does.

Hackmanite shows tenebrescence. This milky white or light pink mineral is often mined in Greenland or Afghanistan. Exposure to UV light typically turns this mineral vivid purple or pink. Sulfur particles play a role in this effect. However, the color and light response can differ slightly from one hackmanite sample to another. That’s because different rocks have slightly different chemical makeups. Other examples of minerals with this property include tugtupite and certain zircon minerals.

Here’s how it works. First, light energizes electrons in these minerals. That means the electrons absorb some of the energy from light. Then, the electrons re-emit this energy as light. That’s where the new color comes from. Importantly, the electrons do not lose their energy immediately. As electrons slowly return to their resting state, the stone retains its new color.

Fluorescent minerals work in a similar way. But there’s a difference. These minerals re-emit their energy almost immediately. That’s why the effect ends when the lights go out.

Tenebrescence can be recreated in the lab. In the 1960s, it inspired scientists to develop sunglass lenses that darken in sunlight.

In a sentence

Minerals’ unique crystal structures may bring many strange properties, such as tenebrescence and phosphorescence.

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