Growing crystals that are in a magnetic field or subjected to an electric field
The main influence on resonant frequency of a crystal is mechanical: the mass of the crystal and the restoring force of elasticity cause the crystal to vibrate at a specific frequency when disturbed. To fine-tune the crystal, bits may be chemically or abrasively removed or the mass may be increased slightly by rubbing with graphite.

That said, crystal orientation does influence resonant frequency because the modulus of elasticity of a crystal such as quartz is anisotropic, i.e. it varies significantly in which plane it is measured. Perhaps applying a strong magnetic or electrostatic field would change the resonant frequency -- but you would need to isolate the effect on the crystal from that of its mounting, particularly if there are metallic contacts.