What element is diamond composed of?

Diamond is composed of the element carbon.

It is an ALLOTROPE of carbon . An ALLOTROPE is a specific arrangement of the atoms on an element,thereby giving it its own physical characteristics.

In diamind any one carbon atom is combined to four adjacent carbon atoms in a tetrahedral arrangement ( like methane). This arrangement tesselates to form a diamond , that you can hold.

The other allotropes of carbon being graphite and Buckminster Fullerene(Footballene).

NB Do NOT confuse the Words 'Allotrope' with 'Isotope'. Within chemistry they have totally different meanings.