So I could not help myself and just had to sate my curiosity and got one of the "real" looking eggs from our favourite auction site. I took some close up photos. It's pretty heavy. Not resin, can't burn or melt it and doesn't release any smells.
Any other tests I can do or visual cues? It has a lot of grey-whitish on remaining eggshell and thinking it could be a morter they rubbed on.
Ideas? Advice on tests?