Chromium oxide stones

good for razors?

This depends on the grit.
Chromium oxide is only a substance which is harder than steel. The size of the particles in the stone will dictate the grit and thus the effectiveness of the stone as a razor hone. Did you mean any stone in particular (manufacturer and model)? I've only ever seen one and it was in the 4000 grit range.

my friend has one that's part of a three piece set, and we don't know if it's suitable for his razor or not. Guess we'll have to check

Most CrO is used as a paste on a paddle strop or hanging strop. I've never seen a CrO hone before.

i've never heard of a chromium oxide hone either. awhile ago i asked about the bonding agents used in making hones when i had the idea that one could potentially use a crapload of chromium oxide and bonding agent to make a very very very fine hone...never tried it though