What is the chemical formula of a diamond?
The answer is simple:
Diamond is one form of carbon; the other is graphite. To distinguish them, we write:
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