There isn't a stone that evolves it. It evolves naturally at lvl 28 into a Primape.
Mankey evolves into Primeape at level 28.
Mankey evolves at level 28 into Primeape.Mankey evolves into Primeape at level 28.
Mankey evolves into Primeape at level 28
mankey evolves into a primeape on level 28
Mankey evolves at level 28.