tenebrescence

tenebrescence (usually uncountable, plural tenebrescences)

1. Reversible photochromism; the ability of a material to change colour when exposed to sunlight.
   • 1954, The American Mineralogist, volume 39, page 616:

     Most fundamental investigations of tenebrescence have dealt exclusively with the alkali halides, since, in addition to their relative simplicity, they were readily available in large single crystals of high optical purity. The reversible discoloration of alkali halides was originally observed by E. Goldstein late in the last century.

• tenebrescent