  Massive gravity is holographically dual to `realistic' materials with
momentum relaxation. The dual graviton potential encodes the phonon dynamics
and it allows for a much broader diversity than considered so far. We construct
a simple family of isotropic and homogeneous materials that exhibit an
interaction-driven Metal-Insulator transition. The transition is triggered by
the formation of polarons -- phonon-electron quasi-bound states that dominate
the conductivities, shifting the spectral weight above a mass gap. We
characterize the polaron gap, width and dispersion.
