What element is composed of diamond?

You misunderstand. The question should read 'From which element is diamond formed'.

The answerr is 'Carbon'.

Diamond is an ALLOTROPE of carbon . An allotrope of an element is where it displays different physical characteristics.

In the case of diamond, any one carbon atom is combined to four adjacent carbon atoms, in a tetrahedral arrangement (like methane). This tetrahedral arrangement of carbon atoms tesselates to form the 'adamantine' structure, which becomes so large that it forms the mineral 'diamond', which you can hold.

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

NB Do NOT confuse the Words 'Allotrope' with 'Isotope', they have two different menaings.