I've tested this a little in duels, using the Crystallized Slab morph, and things are a bit odd to say the least. Crystal Fragments would eat a charge and deal 0 damage to me, but still knock me down. Other abilities, like Destructive Touch (with a Fire Staff, at least), would hit me as though the shield didn't exist at all, while still eating charges from it. In fact, there were numerous times where a single ability would eat all 3 charges in one cast.