  For some symmetric pyramids of $R^3$ , we find Galois obstruction for their
Dehn invariant to be zero, i.e. for the pyramids to be scissor equivalent to a
cube. These conditions are that some associated Kummer extensions of number
fields must be abelian. This work can be viewed as a complement to [5]. Indeed,
in [5], a classification of all rational tetrahedra is given, while we
concentrate our attention on much more symmetric pyramids and prove that the
only ones which are scissor equivalent to a cube are the rational ones.
