At least in my experience just ‘being in combat’ doesn’t seem to cause the ruined ore. Generally, zipping around Orr with mobs beating on me as I harvest, the only time I get greys is if: the mob uses some type of CC on me /and/ I also have an effect on that causes the CC to be instantly negated (aka auto-stability or auto-shake it off from warrior traits). When that happens, my harvesting tool is replaced by my weapon, but still doing the harvesting animations. I guess greatswords make lousy picks/axes?