The arm this affects is chosen at casting time.

Fist of Stone [8] 
    Arm ST (One Arm) 1 [2.1] 
    DR 5 [5] 
        Partial (Affected Arm's Hand) [-80%] 
    Blunt Claws [0.9] 
        Hand on Affected Arm Only [-40%] 
    Magical [-10%] 
    Spell Components [-20%] 
        Must say "stone" in Latin 
        Must gesture with hands 
Levels 2+ [+2.1/level] 
    Increase Arm ST by 1 level [+3 base]