Like many things in Pathfinder, rock trolls are an attempt to reunite and reconcile the folkloric origin of a monster with the stat block it has since become. Trolls in Scandinavian folk tales were petrified by the sunlight, ditto most trolls in fiction (Tolkien, anyone?). Regenerating, fire-fearing trolls are an RPG thing, apparently inspired by Poul Anderson’s Three Hearts and Three Lions (which I’ve never read but have many times heard tell of its influence in D&D).
Rock trolls lose the classic RPG vulnerability to fire, trading it for sonic and sunlight vulnerability instead, which is more in line with the trolls of legend. And that’s…pretty much all you need to know really. They’re a hair more powerful and a hair less bright than the standard troll, so if you wanted a mythologically correct game you could even just make rock trolls your default troll entirely without disrupting play. You also might play a bit with the crystalline content of the trolls’ skin—it might change depending on their region of origin or recent diet, reflecting the stones they’ve eaten. Other than that, they’re just another fine troll subspecies with which to surprise spelunking PCs and frustrate fireball-tossing mages.
An elderly xorn wants to consume the tourmaline-studded skin of a live rock troll—a delicacy. But in its infirmity it needs guides to help it find one of the creatures, as well as help hacking the regenerating thing apart if the party can’t find one small enough for the xorn to gulp whole.
Dwarven bards from all over the continent are gathering at Caer Undwen to trade songs, sing dirges, and tune their instruments to the perfect pitch of the Cavern of the Spheres. Unfortunately, this leaves no one with musical skill remaining at the dwarf settlement of Holdfast. When the clan unearths a gang of hibernating rock trolls, there is no one to take hammer to the chimes of shattering that would wound the creatures. Without the help of brave adventurers, the clan is doomed to days of tunnel fighting and attrition in the depths.
Necromancers are notoriously hard on apprentices, but Maxim of the White Hand is worse than most. If adventurers enter the courtyard of his isolated keep, he sends one of his pupils to greet them and offer refreshments…while another, invisible, uses scrolls of stone to flesh to awaken the two rock troll “statues” in the courtyard. If the apprentices are killed in the process (the trolls do not discriminate), Maxim reanimates their bodies later. What’s left of them, anyway…
—Pathfinder Bestiary 2 272