Is diamond a compond or a element?

Diamond is a MOLECULE of the element Carbon only. It is NOT a compound.

Compounds are molecules of the atoms of different elements e.g. water (H2O) is a compound. Two different elements 'H' & 'O' , but oxygen (O2) is a molecule. Only one element 'O' present.

The distinction between molecules and compounds is very subtle.

Molecules ; A combination of the SAME or DIFFERENT elemental atoms'

Compound ; A combination of DIFFERENT elemental atoms'

Diamond is composed of the element Carbon only. The carbon atoms are held in a tetrahedral arrangement, known as an 'Adamantine Structure'.