These look a lot like quarry tiles with a huge markup. 3/4" quarry tiles are better than the usual 1/2" and 1/4" that you find, but, you're still talking about extremely low conductivity and long-ish (more than 8 minute) bakes.
Their darker color makes them good candidates for ceilings, but, for hearths, quarry tiles are about the worse stone you can buy- and to charge about 4 times what they should cost, that's disgraceful.