nigrescite
/nɪˈɡrɛskʌɪt/Mineralogy rare
noun
An amorphous hydrated silicate of magnesium, ferrous iron, calcium, and aluminium, which is green when first exposed to the air but gradually turns black.
Origin
Late 19th century; earliest use found in Edward Dana (1849–1935), mineralogist and physicist. From German Nigrescit from classical Latin nigrēscere to grow black + German -it.