Garnet is a common mineral in medium to high grade metamorphosed pelitic and mafic  rocks.  The characteristic high positive relief of garnet relative to its surrounding minerals is obvious in A, in which two adjacent subhedral garnet porphyroblasts are set in a matrix dominated by quartz, muscovite and biotite.  Garnet is isotropic, and thus appears black under crossed nicols (B), and the quartz inclusions have birefringence.  The relief of garnet in C is much less than in A, because the surrounding mineral in C is clinopyroxene, which has refractive indices closer to that of garnet than does the low-relief quartz in A.  Under crossed nicols in D, the garnet is isotropic, though the presence of secondary calcite along inclined fractures shows as patches of first order colours.  The fractures appear as grey areas in C.  A and B are from a garnet mica schist from an unknown location and C and D are from a mantle eclogite xenolith from the Kaalvallei kimberlite, South Africa.  All views are 5.5 mm across.  A and C ppl, B and D x-nicols.