For a neutral carbon atom in its ground state, the mass is

The unified atomic mass unit ($1 \text{ u}$) is defined as exactly $\dfrac{1}{12}$ of the mass of an unbound, neutral carbon-12 atom in its nuclear and electronic ground state.

Therefore, the mass of a standard neutral carbon atom (carbon-12) in its ground state is exactly $12 \text{ u}$.

The mass of an individual atom is an intrinsic property and does not depend on the macroscopic allotropic form (such as graphite, diamond, or charcoal) it may be a part of.