Arsenopyrite and covelline. Cligga Head, Cornwall, Britain
Characteristic rhombic crystals of arsenopyrite (white, high reflectance, centre) occur within quartz (low reflectance, bottom centre) and the main gangue phase, tourmaline, which shows bireflectance (greys, centre). Banded covelline (deep blue, top left) has extensively replaced a large arsenopyrite crystal. Black areas are vugs and polishing pits.
Polished block, plane polarized light, x 80, air