It's just making it into a different type of rock, change which quarry it came from or just a mix up with the builders who made the room, seems pretty plausible, plus if it changes back into stone after you carve it that's even better, it'd be harder
EDIT:
Also creating a soul stamp for a guard you don't know would be nearly impossible, Shai is one of the best Forgers alive and only just manages to pull it off when she has access to infinite stamps, people who know the subject well enough to talk, his journals, and test subjects. It also takes more than one stamp.