Corundum is restricted to silica-undersaturated rocks high in aluminum and high-grade pelitic metamorphic assemblages.  It has very high positive relief relative to feldspars, and is never found coexisting in equilibrium with quartz.  In A, corundum has a thin rim of nepheline, and stands out in positive relief relative to the nepheline and associated feldspars (which are first order grey to white in image B).  Images C and D illustrate subhedral crystals of corundum in a mantle eclogite xenolith (dominated by garnet and altered clinopyroxene).  The typical high positive relief of corundum is not evident in C due to the high refractive index of surrounding garnet and altered clinopyroxene. Twinning, a common feature of corundum, is present in most of the grains shown in C and D.  Corundum is uniaxial and elongate grains have parallel extinction.  The interference colours of first order orange in B and first order yellow in D are just above the maximum of first order white typically expected.  Note that due to the extreme hardness of corundum (9 on Mohs scale), corundum in many thin sections is not ground down to the expected 30 microns of a standard thin section.  A and B are from a corundum syenite from an unknown location and C and D are from a corundum eclogite xenolith the Bobbejaan kimberlite, South Africa.  All views are 5.5 mm across. A and C ppl, B and D x-nicols.