Stone Knights move slowly, and pause before firing their projectiles, so their offense is somewhat lacking. They make up for this in hardiness, though. It takes three sustained blows with the Long Sword to defeat a Blue Stone Knight (but only one with the Valley Sword!).
White Stone Knights have an immense amount of health, and deal an astounding 7 damage with their touch. They are still slow and easy to avoid, but fighting them without the Legendary Sword can be tough. At this point in the game, their weak projectiles are probably not a significant concern.