Forms of Carbon

Diamond

Diamonds are made up of carbon atoms joined with four other carbon atoms with a strong covalent bond without any freely moving electrons or ions making them extremely hard and having a high melting point. Not only that they are unable to conduct electricity because all four electrons in an atom are shared to four other carbon atoms.
Because diamonds are very hard, they are used in many oil drilling industry.

Graphite has layer of carbon atoms instead of all atoms being connected, the carbon atoms are held with strong bonds while the layers are held with weak bonds, because of their structure they are softer than diamonds. The carbon atoms in the layer bonds with three other carbon atoms leaving one freely moving electron in each atom, making a sea of moving electrons making them able to conduct electricity well.
Graphite is slippery so it is used in making pencil leads because it can slip easily unlike diamond which is sharp.

Graphite has layer of carbon atoms instead of all atoms being connected, the carbon atoms are held with strong bonds while the layers are held with weak bonds, because of their structure they are softer than diamonds. The carbon atoms in the layer bonds with three other carbon atoms leaving one freely moving electron in each atom, making a sea of moving electrons making them able to conduct electricity well.
Graphite is slippery so it is used in making pencil leads because it can slip easily unlike diamond which is sharp.