"wiki: Increases mining speed +30% over the previous level: I=130%, II=169%, III=220%, IV=286%, V=371%. "

it does not work by "Hardened Clay"
if I mining hardened Clay wit a Diamond Axe with "Effiency V" it is a little bit faster than with a normal Stone Axe.
Mining Sandstone are very fast with the same Dimond Axe EffV.