Perhaps it was simply stored with something minty, like inside an Altoids tin?Pipes becoming “minted” after an overzealous hit in which the flame is (drawn in via inhalation) on it for too long, and past the point when any remaining rock has either combusted or run through your brillo or ashes, or whatever. This often results in a sour wintergreen-esque odor, and bitching from whomever else is using the pipe.
If your rock smells minty, my suspicion is that someone re-rocked it and added crushed Altoids or some other similar breathmint or minty candy.