Original: On the Mohs scale, a pencil "lead" (graphite) has a hardness of 1; a fingernail, 2.2 - 2.5; a copper penny, 3.2 - 3.5; a pocketknife 5.1; a knife blade, 5.5; window glass plate, 5.5; and a steel file, 6.5.
Simplified: 5.5 Knife blade