✺ UV Reactive
Under a black light some minerals glow in neon colours. Trace elements and defects in the crystal absorb invisible ultraviolet light and give it back as visible colour.
Fluorescence
A glow that lasts only while the UV light is on. Colours often differ between longwave (~365 nm, common LED flashlights), midwave (~310 nm) and shortwave (~254 nm) lamps.
Phosphorescence
An afterglow that continues for a moment, or longer, after the lamp is switched off. Willemite and Franklin calcite are classic examples.
Tenebrescence
A reversible colour change caused by light. Hackmanite and tugtupite deepen in colour after UV exposure and fade again.
Franklin & Sterling Hill
The zinc mines of Franklin and Sterling Hill in New Jersey are known as the "Fluorescent Mineral Capital of the World", famous for red calcite and green willemite. Franklinite, the ore mineral named after the town, does not fluoresce.
⚠️ Shortwave UV can burn skin and damage eyes. Wear UV-blocking glasses, and never look into any UV lamp. Reactions vary by locality, and many specimens of a "fluorescent" mineral don't glow at all.
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