Why Is Diamond Hard and Pencil Lead Soft?

Step 1 of 5

Both are pure carbon. Nothing else.

There is no ingredient in a diamond that is missing from a pencil. Burn either one in enough oxygen and you get the same gas out.

Step 2 of 5

In diamond, every atom is tied to four neighbours.

The bonds point out in all directions, so the whole stone is a single net. To scratch it you have to break bonds, not just slide something.

Step 3 of 5

In pencil lead, the atoms lie in flat sheets.

Inside each sheet the bonds are extremely strong. Between the sheets they are feeble, so the sheets slide off each other at the lightest touch.

Step 4 of 5

Writing is sheets being left behind.

Drag a pencil across paper and you are shearing off layers of carbon and leaving them on the fibres. The grey line is a stack of slid-off sheets.

Step 5 of 5

Hardness is really about which bonds have to break.

Diamond scratches everything because every direction is a strong bond. Graphite marks everything because one direction has almost no bond at all.

Turning graphite into diamond takes about 1,500°C and 50,000 times the pressure of the air around you. Deep underground both happen at once, which is where natural diamonds come from.