A pencil mark actually consists of graphite particles abraded from the pencil point by the paper. These particles, which have an angular, gritty look under the microscope, are for an HB lead pencil, typically between 2 and 10 micrometers in diameter. The particles lie slightly below the surface of the paper, interlocked between its fibres.
A signal rub using a rubber sufficiently soft to reach between the fibres will pick up most of them. Inspection of the rubber shows the undamaged particles adhering to the surface.
An effective erasing material is also abraded by the paper surface, producing the familiar small spindles of rubber or eraser material, which wrap up the graphite particles. At 200 x magnification, these look like roly-poly puddings studded with graphite raisins.