  We propose and demonstrate a test of Lorentz symmetry based on new, compact,
and reliable quartz oscillator technology. Violations of Lorentz invariance in
the matter and photon-sector of the standard model extension (SME) generate
anisotropies in particles' inertial masses and the elastic constants, giving
rise to measurable anisotopies in the resonance frequencies of acoustic modes
in solids. A first realization of such a "phonon-sector" test of Lorentz
symmetry using room-temperature SC-cut crystals provides a limit of $\tilde
c_Q^{\rm n}=(-1.8 \pm 2.2)\times 10^{-14}$\,GeV on the most weakly constrained
neutron-sector $c-$coefficient of the SME. Future experiments with cryogenic
oscillators promise significant improvements in accuracy, opening up the
potential for improved limits on Lorentz violation in the neutron, proton,
electron and photon sector.
