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Well if it was celluloid or something similar, you would know. I made the mistake of sanding some old scales before reading all the info and I almost choked to death on the stench.

To quote...

It is hard to tell for sure. How do they smell after you rub the scales down?

Looks like horn to me. If epoxy works for fixing the crack, you may be able to use CA (superglue) to seal the scales up, but I'm not sure if it works with Horn or not since I have only worked with...