Tenebrescence

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.

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Tenebrescent minerals include hackmanite sodalite, spodumene and tugtupite.

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