Graphite. Strontian, Scotland
A metapelite with flakes of graphite that show strong reflection pleochroism and bireflectance, from brown-grey (when oriented east-west, top right) to grey (when oriented north-south, centre) where it is almost indistinguishable from the mica. The silicates are quartz, plagioclase (featureless, hard) and biotite, which shows reflection pleochroism from grey (centre left) to brown (bottom left).
Polished thin section, plane polarized light, x 80, air