Biotite is a brown mineral which shows a property called pleochroism.  That is, under a single polarizing filter, the grains change color (from light brown to dark brown) when rotated.  Under both polarizing filters, the mineral shows a high birefringence, but the brilliancy of the stained-glass-like interference colors is masked somewhat by the brown color of the mineral itself.  This pair of pictures shows several distinct biotite grains diagonally across the field of view, with the mineral kyanite at lower left and quartz at upper right.

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