No reason whatsoever. Just "problem". Well as it isn't "antik" or precious, I can only guess that a 3"x3" stone could be used as a weapon, I could take over the plane with a heart shaped implement.
The large bottle of water in the same bag, still frozen pretty much as it came out of the freezer as we left for the airport would allow a far greater swing and possibly inflict far greater injuries, but that was not a "problem".