Probably Soapstone. Tough to photograph; cabochon is slightly lighter than it appears. I'm going to keep this one for the collection.

37mm x 16mm x 6.5mm; cab has a flat girdle.   Weight: 29.75 carats.

Click the image on the right to see the cabochon super-sized...

This cabochon is in my personal collection. It is not for sale.