Polarized light is depolarized at the interface between a conductive particle and a non-conductive mounting medium. This light halo effect with transmitted crossed polarized light indicates an opaque particle is a wear metal particle or at least is conductive. Graphite is sufficiently conductive to produce this effect. Pencil debris can be distinguished from combustion residue by this effect.

Fretting Metal Wear

Graphite

Magnetite Spheres