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

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

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.