Tourmaline is a common accessory mineral in plutonic felsic igneous rocks (especially evolved varieties such as pegmatites) and meta-pelites.  It has moderately high positive relief relative to quartz and feldspars.  In coarse crystals its trigonal prismatic form is commonly obvious.  Euhedral to subhedral triangular cross-sections in A and B represent sections cut approximately perpendicular to the “c” axis, and colour zonation is apparent in the larger of the two (which encloses several quartz grains).  The different intensity of yellow of the elongate grain in views A and B (which are oriented at right angles to one another) illustrates the differential absorption of tourmaline.  Unlike most pleochroic minerals (e.g., biotite), tourmaline appears darkest when the long axis of the crystal is oriented at right angles to the polarizer.  (The polarizer is east-west in these images.)  Tourmaline absorption colours cover a wide range, including some varieties that are so dark as to appear nearly opaque in some orientations.  Extinction is parallel to the long (“c”) axis, and the elongated grain in C, oriented at a 45 degree angle to images A and B, illustrates the typical maximum interference colours of mid-second order, colours that are approximately the same as the surrounding muscovite in this view.  Tourmaline in D exhibits blue absorption colours which can be seen in several grains with c axes approximately horizontal, but the grains with c axes approximately vertical appear virtually black due to the strong absorption in this orientation.  Samples are from an unknown location.  All views are 2.2 mm across.  A, B, D ppl, C x-nicols.