The property due to which an element exists in two or more forms which differ in their physical and some of the chemical properties is known as allotropy and the various forms are called allotropes or allotropic modifications. Carbon exists in two allotropic forms crystalline and amorphous. The crystalline forms are diamond and graphite.

Diamond due to extended covalent bonding is the hardest natural substance on the earth. Graphite has layer of sheets which are held by weak van der Waals forces thus, it can cleaves easily between layers which makes it soft and slippery.