A real shell will be "hollow" all the way to the apex.  A fake will only
be hollow for a whorl or two, with most of the spire filled solid. I
wonder if the shell is sufficiently translucent to check this using
bright backlighting, rather than x-ray?  This is one reason I was
wondering what they are made of.  It must be something pretty
lightweight.  Otherwise the weight of the solid specimen would be a dead giveaway.

Paul M.