Tenebrescence

Tenebrescence, also known as reversible photochromism, is the ability of minerals to change colour when exposed to light. The effect can be repeated indefinitely, but is destroyed by heating.[1]

Hackmanite before (top) and after (bottom) exposure to UV

Tenebrescent minerals include hackmanite, spodumene and tugtupite.

Tenebrescent behavior is exploited in synthetic materials for the manufacture of self-adjusting sunglasses, which darken on exposure to sunlight.

References
  1. Kondo, D.; Beaton, D. (2009). "Hackmanite/Sodalite from Myanmar and Afghanistan" (PDF). Gems and Gemology. 45 (1): 38–43.
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