And it can't be exceptional- has to be "normal" quality and normal color, and is simply the "base" plus 15,,, if you want it to be higher than the base +15 you have to enhance it AFTER you imbue it, then the extra resist bonus from the various colors can add or stack on it,, but tht is a rather risky and expensive proposition because of the extremely high failure rate on enhancing.
For example in ur case no matter if u make the pc from normal wood OR heartwood, it still has a base resist of 5,3,2,3,2 and can be max imbued up to 20,18,17,18,17 BUT you could make it from the normal wood, max imbue it, and THEN enhance with the heartwood and get a resist of 22,21,19,25,19, IF and a mighty big IF you are successful upon attempting the enhancing.