Graphite is a friable mineral which has been used for centuries for writing (Indian ink, "lead" pencil).
The structure of graphite is a stacking of planes, each being made of a regular honeycomb pattern of hexagons.
Each carbon atom is bound to three neighbouring atoms in the hexagon plane by bonds at an angle of 120° from each other. These plane bonds are strong and have a distance of 0.142 nm between atoms.
On the other hand the atoms are weakly bound to the atoms of neighbouring planes with a distance of 0.34 nm between planes.
This structure has a density three times lower than that of diamond and makes graphite a very anisotropic, virtually 2-dimensional solid because the weakly bound planes slide easily over each other.
The structure of graphite, from the Greek "graphein" = to write ("lead" pencil, Indian ink).