Fluorite is a fairly common mineral in hydrothermal veins and occurs as an accessory mineral in some granitic rocks.  The perfect {111} cleavage is visible in A in the euhedral to subhedral fluorite cubes in a matrix of barite. Fluorite is isotropic and viewed under crossed nicols it is black (extinct), as in C, in which it is intergrown with high-order white calcite.  It has the lowest refractive index (n ~ 1.434) of all the common minerals and typically stands out with high negative relief, as seen relative to barite in A and calcite in B. Fluorite can be coloured in thin section (blue and purple are common colours) and in images B and D the purple colour is unevenly distributed.  In D the intensity of the colour increases towards a small inclusion of uraninite (partially plucked out during thin section preparation) and the origin of colour of the fluorite from this locality is thought to be due to radiation damage.  A is from the Noyes Mine near Madoc, Ontario and B, C and D are from the Richardson (Fission) Mine near Wilberforce, Ontario.  View A is 5.5 mm across and B, C and D are 2.2 mm across.  A, B and D ppl and C x-nicols.