In capacitors dielectric materials are sometimes inserted between the parallel plates. What would happen if a diamagnetic, or a paramagnetic material is inserted between the parallel plates of a capacitor?
As long as neither diamagnetic or paramagnetic material acts as a conductor (no flow of charge between plates below breakdown voltage), it will still behave as a capacitor.
The dielectric polarisability of the material will however have an influence on the total storage of charge and charging profile of the capacitor, as the material can store additional charge through polarisation under the applied field. The degree and nature of this influence will depend on the specific nature of the material.