Both Diamond & Graphite (the “lead” in pencils) are made of pure Carbon. So why are they so different in both cleavage, hardness, and density? This can be explained by the Internal Atomic Structure. Minerals are made up of atoms (microscopic particles) bonded together in a regular geometrical pattern. In the photos, I used Zomes©(We got the Zomes© on Dad’s birthday years ago) for examples. Now, lets pretend the little white balls are carbon atoms, and the “sticks” are the bonds. Now photo #1 is diamond, each atom is very strongly bonded to four others (the high hardness), above and below forming a compact structure (the low cleavage and high density). Photo #2 is graphite, each atom is bonded to only three others,(the low hardness) making thin plates of hexagons. Each plate being very widely bonded together (low density), the plates easily slip over each other (high cleavage).