This was kind of a dare, really.
The Pebble is a triangular stone of unknown type that is about three inches in length, and most notable for two things. The first is the white line that scores one-third of the Pebble. The second is the way that moving the Pebble leaves behind a white line of force in the air that seems to be functionally equivalent to a monomolecular blade. Worse, the line persists for at least a day before it finally fades. Or, possibly, the universe heals the wound being made in it.