Graphite

Chemical Formula : C
Hardness : 1-2
Cleavage : One; perfect and easy
Luster : Metallic to dull
Color : Black to gray
Streak : Black
Density : 2.09-2.23
Diagnostic Features and Trivia : Graphite's low hardness and ease of cleavage is what makes it so useful as penicil lead. Various methods of pressure treating allow for varying bond strength which alters the hardness and therefore darkness of the "lead."
Crystal System and Habit : Hexagonal; uncommon - tabular
Twinning : None
Polymorphs : Diamond
Index of Refraction : Opaque