Not sure, but I think this is Poppy Jasper. It appears to be the same material as this cabochon. Somewhere in its very distant past, this stone cracked and was eventually filled with Calcite. You can see the Calcite as a bold white stripe across the cabochon. The cab is very stable and has a unique look.

Dimensions: 37mm x 27mm x 6mm.  Weight: 39.05 carats.

This cabochon is in my personal collection; it is not available for sale.