I understand that a "stone" is a unit of weight for a person, though it is not a unit I've ever seen used in certification of aircraft. The design requirement for a seat is 170 pounds, unless that seat is placarded for a lesser weight (the kiddy seat in the back of a 150/152 being an example of a lesser occupant weight being placarded).
Ironically, the weight of the occupant of a seat is not generally "limited" to 170 pound, or any other weight, unless specifically placarded. Every other compartment or restraint device will have a limiting maximum capacity - except a seat, unless placarded. You are still bound by the whole aircraft weight and C of G limits though. Quietly, regulators admit that they just don't want the political minefield of limiting occupant weight. Those occupants should know though, that the heavier they are the less protection/security they have in a crash.
So, if you would like to carry a strapping lad/lass, and the aircraft does not have a placarded limitation for a seat either on/near the seat, and in the flight manual limitations section, and you can fit them in, you are permitted to fly them - bu YOU must be content that it's safe.