It's what's known as a polymorph,

meaning it can exist in more than one form, or crystal structure.

Each form is called an allotrope.

The best example for Carbon

is graphite

and diamond.

Allotropes occur when the atoms bond together in different ways.

Watch to see the difference between

a graphite and diamond structure.

The bond for graphite atoms are weak

making it easy to break.

But diamonds are bonded together strongly.