Idocrase, also known as vesuvianite, is common in contact metamorphosed limestones.  It has high positive relief relative to quartz, for example (A, B).  It is colourless or pale yellow to brown and can be faintly pleochroic (e.g., colourless to yellow in A and B, rotated 90 degrees to one another).  Idocrase usually occurs as anhedral grains or stubby prisms (with parallel extinction) and poor cleavage.  Interference colours are typically lower first order (C) and are commonly anomalous due to dispersion (D).  A-C from an unknown location and D is from the wollastonite-bearing Valley Springs Gneiss, Llano, Texas.  All fields of view 2.2 mm across.  A, B ppl, C, D x-nicols.