Molybdenite. Henderson Mine, Colorado, USA
Stubby crystals of molybdenite show cleavage along (0001) (centre) and marked bireflectance (grey to grey-brown). Lath-shaped crystals are well polished but basal sections are poorly polished (centre). The gangue is quartz showing diffuse internal reflections. Black areas are polishing pits.
Polished block, plane polarized light, x 80, air