  The standard view is that at low energies Mott insulators exhibit only
magnetic properties while charge degrees of freedom are frozen out as the
electrons become localized by a strong Coulomb repulsion. We demonstrate that
this is in general not true: for certain spin textures {\it spontaneous
circular electric currents} or {\it nonuniform charge distribution} exist in
the ground state of Mott insulators. In addition, low-energy ``magnetic''
states contribute comparably to the dielectric and magnetic functions
$\epsilon_{ik}(\omega)$ and $\mu_{ik}(\omega)$ leading to interesting phenomena
such as rotation the electric field polarization and resonances which may be
common for both functions producing a negative refraction index in a window of
frequencies.
