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]

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.