Currently it seems that all White Hand gems have no stats other than a proc of the type “chance on damage, … on target”. This type of proc only triggers when you do physical damage to your enemy, not when you do spell damage,* so these gems are all useless to casters. What exactly are casters supposed to put in that gem slot then?
[*And it has always been thus. If you look at old-world blue weapons that sometimes have procs, you find “chance on damage” procs only on weapons which, as is clear from their other stats, were intended for non-caster classes, while weapons intended for casters have no procs at all - there is no “chance on spell damage” proc or anything of that sort.]