Seastone is a type of mineral. It exudes the energy of the ocean so all devil fruit users are weak to it. My jitte was tipped with it, but I can't seem to find it here. Haki is... I think what you were describing for. Sort of a lifeforce energy channeled outward. Gold Roger -- the last pirate King, was infamous for using it with great efficacy. When it's strong enough it cuts through logia.