Titanite, also known as sphene, is a common accessory mineral in many igneous and metamorphic rocks, where it is typically the dominant carrier of titanium.  In plane light, titanite is colourless to pale brown and has very high positive relief relative to quartz and feldspars (A, C).  The classic diamond cross section of euhedral grains is evident in A and B, and in this orientation titanite has symmetric extinction.  Titanite has extreme birefringence (high order white, similar to calcite) apparent in B and D, and many grains look similar in plane light and viewed under crossed nicols.  A and B are from a granite from the Hepburn Batholith, N.W.T.  and C and D are from an essexite from Mt. Johnson, Quebec.  All views are 5.5 mm across.  A and C ppl, B and D x-nicols.