  Single-crystal diamond cavity optomechanical devices are a promising example
of a hybrid quantum system: by coupling mechanical resonances to both light and
electron spins, they can enable new ways for photons to control solid state
qubits. However, realizing cavity optomechanical devices from high quality
diamond chips has been an outstanding challenge. Here we demonstrate
single-crystal diamond cavity optomechanical devices that can enable
photon-phonon-spin coupling. Cavity optomechanical coupling to $2\,\text{GHz}$
frequency ($f_\text{m}$) mechanical resonances is observed. In room temperature
ambient conditions, these resonances have a record combination of low
dissipation (mechanical quality factor, $Q_\text{m} > 9000$) and high
frequency, with $Q_\text{m}\cdot f_\text{m} \sim 1.9\times10^{13}$ sufficient
for room temperature single phonon coherence. The system exhibits high optical
quality factor ($Q_\text{o} > 10^4$) resonances at infrared and visible
wavelengths, is nearly sideband resolved, and exhibits optomechanical
cooperativity $C\sim 3$. The devices' potential for optomechanical control of
diamond electron spins is demonstrated through radiation pressure excitation of
mechanical self-oscillations whose 31 pm amplitude is predicted to provide 0.6
MHz coupling rates to diamond nitrogen vacancy center ground state transitions
(6 Hz / phonon), and $\sim10^5$ stronger coupling rates to excited state
transitions.
