The answer is simple: ##”C”##.
Diamond is one form of carbon; the other is graphite. To distinguish them, we write:
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diamond: C(s,diamond)
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graphite: C(s,graphite)
The s stands for solid.
Both diamond and graphite are allotropes of carbon. There are other exotic allotropes of carbon (graphenes and fullerenes among them) but they are much less common.
A diamond consists of a giant three-dimensional network of carbon atoms.
In effect, a diamond is one giant molecule. All we can do is write its empirical formula, which is ##”C”##.
