diamond (all carbon)

diamond is a allotropic form of carbon.

Hence it is a compound which is made up of one kind of atom only. It cannot be an element, becuase element are those on the periodic table.

A chemical element, or element for short, is a pure substance that cannot be decomposed into any simpler substance.[

The formula of diamond is (C)n..there are n C atoms.

diamond (all carbon) 係compound，因為佢由碳化合而成，其生成物（鑽石）無保留碳含有的性質。（其實即係問緊compound既definition）
element既definition係element只由一種原子組成，而且不能以化學方法分解的純物質。