u/Flat-Incident5639

Approximately 6 x 5 inches (two pics show ruler for scale), and weighs a little over 11 pounds. I cannot scratch it with a fingernail, but it does scratch with a pre-1982 penny. A drop of vinegar on its surface shows no chemical reaction.

Any ideas on what this could be? We picked it up at an estate sale, so there is no information on what its source might be.