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Yup, rub venously with your thumb and smell. People used to complain that Dentist offices smelled of burnt rubber, it was and still is the filling use to pack a tooth root for root canals.

If they...

Probably Gutta Percha, popular in the early 1900’s for razor, knife scales and gun grips. Also, for telephones, and why they were black.

Polishes well, breaks easily, and cannot be glued.