Covelline in Pyrite – Tilva Mika Orebody, Bor, Serbia
Found in 1942
 
Indigo blue covellin in a pyrite-containing matrix. The thin leafed crystals can partly be seen.
Covellin has unusual optical properties. Due to its high dispersion, the color of the mineral changes greatly depending on the medium in which it is located. The dry mineral appears blue when exposed to air; when wetted with water, it appears violet. If you immerse it in an oil that is highly refractive, Covellin will eventually appear red.
Historical find from 1942.