Kyanite is the aluminosilicate polymorph stable at intermediate grades of metamorphism, especially in regionally metamorphosed pelitic rocks, but it is stable to high pressures and is well-known from mantle eclogites.  Kyanite has high relief relative to coexisting quartz, plagioclase and muscovite, with which it is commonly found in pelitic metamorphic rocks (A, C).  It is typically colourless, or rarely pale blue, in plane light.  Two cleavages are visible in some grains, although the {100} cleavage is typically dominant, and is obvious in several grains shown here.  Twinning on {100} is common (D).  The maximum interference colour of first order red is just slightly above the maximum of first order yellow-orange in B and D. Kyanite has a small extinction angle (30 degrees), and can appear to have parallel extinction.  A and B are from a kyanite schist from Fernleigh, Ontario and C and D are from an unknown location.  All views are 2.2 mm across.  A and C ppl, B and D x-nicols.