I have an epicyclic gear complete with bearings on my desk, and given the ongoing discussion and it's in-visual line to my computer screen I have just decided to satisfy my curiosity to see if it it will break when hit with a hammer.
I don't know what alloy it's made of or what they do to harden it but it didn't break and I don't recommend anyone doing it either.
It does however satisfy my curiosity about this ongoing discussion and a personal theory that if it was going to break easily it would have done so years ago from the abuse it regularly receives by 'accidentally' falling on the floor due to enthusiastic dusting.