Sphalerite, the dominant zinc ore,  is common in hydrothermal ore deposits, where it is commonly associated with galena and other sulphides.  In A, euhedral to subhedral yellow-brown sphalerite grains stand out in extreme positive relief relative to intergrown dolomite euhedra.  Refractive index and colour intensity increase with increasing iron content, and in B colour zonation indicates variable Zn:Fe ratios.  Sphalerite is isotropic.  Some of the grains in B show evidence of the perfect dodecahedral cleavage.  Sample is from the San Vincente mine, central Peru.  Both images are 2.2 mm across and ppl.