I'm not a pro at heating, but I do heat a lot of my stone. I would saw or knap that into thinner pieces before heating it. As dark as that is (rootbeer?), I would take it up to 400-425 slowly and then hold it there for at least 12 hours. Every person heat treating rock will give you a different recipe. I doubt that rock can take more than 425 F, and then it needs to be thinned into slabs or preforms before heating.
WA