  Optically isotropic metamaterials (MMs) are required for the implementation
of subwavelength imaging systems. At first glance one would expect that their
design should be based on unit cells exhibiting a cubic symmetry being the
highest crystal symmetry. It is anticipated that this is a sufficient condition
since it is usually assumed that light does not resolve the spatial details of
MM but experiences the properties of an effective medium, which is then
optically isotropic. In this work we challenge this assumption by analyzing the
isofrequency surfaces of the dispersion relation of the split-cube in carcass
(SCiC) negative index MM. We show that this MM is basically optically
isotropic, but not in the spectral domain where it exhibits negative
refraction. The primary goal of this contribution is to introduce a tool that
allows to probe a MM against optical isotropy.
