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.
Top of "Enjoying Rocks Under the Microscope"