My reading of this, and if I'm being overly pedantic, feel free to say...
I don't think this is a spell to quarry rock. I think this is a spell to shape a rock into a finished piece suitable for building. If you want to quarry rock, I'm inclined to say it needs to be something like the Bone Carver of Atlas. That will allow someone to quarry the material, for it to be shaped later.
Again, I wouldn't worry about Viscaria's finesse problem too overmuch. I have something in mind for it. Because I had mentioned that earlier, I didn't mention it again. If you want Viscaria's pater to have taught her this spell, I'm fine with that. Sometimes masters do things for reasons their filia can't fathom. Indeed, that's the entire basis for Talia of Tytalus in PB's Canaries are dying saga. He completely deemphasized her natural talents and taught to her weaknesses. Why? Who knows, he's a Tytalus.