  I discuss elastic properties of neutron star crust in the framework of static
Coulomb solid model when atomic nuclei are treated as non-vibrating point
charges; electron screening is neglected. The results are also applicable for
solidified white dwarf cores and other materials, which can be modeled as
Coulomb solids (dusty plasma, trapped ions, etc.). I demonstrate that the
Coulomb part of the stress-strain tensor has additional symmetry: contraction
$B_{ijil}=0$. It does not depend on the structure (crystalline or amorphous)
and composition. I show as a result of this symmetry the effective (Voigt
averaged) shear modulus of the polycrystalline or amorphous matter to be equal
to $-2/15$ of the Coulomb (Madelung) energy density at undeformed state. This
result is general and exact within the model applied. Since the linear mixing
rule and the ion sphere model are used, I can suggest a simple universal
estimate for the effective shear modulus: $\sum_Z 0.12\, n_Z Z^{5/3}e^2
/a_\mathrm{e}$. Here summation is taken over ion species, $n_Z$ is number
density of ions with charge $Ze$. Finally $a_\mathrm{e}=(4 \pi
n_\mathrm{e}/3)^{-1/3}$ is electron sphere radius. Quasineutrality condition
$n_\mathrm{e}=\sum_Z Z n_Z$ is assumed.
