  We present measurements of the Berry Phase in a single solid-state spin qubit
associated with the nitrogen-vacancy center in diamond. Our results demonstrate
the remarkable degree of coherent control achievable in the presence of a
highly complex solid-state environment. We manipulate the spin qubit
geometrically by careful application of microwave radiation that creates an
effective rotating magnetic field, and observe the resulting phase via
spin-echo interferometry. We find good agreement with Berry's predictions
within experimental errors. We also investigated the role of the environment on
the geometric phase, and observed that unlike other solid-state qubit systems,
the dephasing was primarily dominated by fast radial fluctuations in the path.
