I have almost completed the stout mines. have visited all the track areas and have the dragon ore but where is this living rock that turns you to stone

Consensus has it that it is the lava - there is certainly no big boss for you to kill down there.

I ran into some critters in there called 'Slag' something-or-others (tech term) that looked like mucks or the swamp things at the toad totem grounds. They turn you to stone among other things. Maybe that's what they're talking about.
Mind you, the lava hound is no pushover - he can breath fire continuously if you try to stay at range.

The slime does stuff? I realised early on that I could (with my little ninja) climb up the cave walls in every room at least half way to the roof, so when I encountered the slime I climbed up the wall & Incinerated it; it never did do anything to me. The Lava Hound took more doing, but (once again utilising the wall trick) took almost no damage killing him & the puppies.

I really don't think there is a big boss, and I searched out every cranny in the caves, because I was also killing time waiting for my monk