Ever since I first saw the Stonecast Eternals, I was mesmerized and thought they were absolutely awesome. And I also silently cursed the fate that made me play Necrons in
40k and not something that lent themselves more easily to stone statues in
AoS.
But then again, Necron models wouldn't be the worst of armies to apply this concept to. They certainly have something eternal and ancient to them. Although why living metal would turn into stone takes some imagination. But its not impossible (That extremely rare mineral deep under the crust on this particular planet - over millions of years it slowly interacted with the living metal, slowly turning it into stone etc. etc.)
Luckily the guy published the steps to produce them, and working with decent Vallejo equivalents I came up with what you can see below. Zooming in there are definitely some sloppy lines here and there, and the moss looks messy, but this is only the first, practice model, things will get better.