Stoneskins are Silvers who have the power to transform their skin into stone. This gives them greater durability, but they are still vulnerable to greater amounts of force.
"Stoneskins are perceptors, able to shift their own skin until it hardens into stonelike plates. Lesser stoneskins are more susceptible to harm, but well-bred stoneskins, like those of House Macanthos in Norta, can form all-but-impenetrable armor." -Victoria Aveyard "War Storm" "On Silverblooded Abilities"