A stainless powder metal super steel by Hitachi of Japan. It has a huge amount of carbon and Chromium which combined give it ridiculously high levels of hardness (64-66 Rc!). It has superb edge retention, but of course is very difficult to sharpen. The Carbon and the Chromium pairs up, which doesn't leave a lot of surface Chromium available to fight corrosion, it is still stainless but isn't overly resistant compared to something like S30V. It is a very expensive steel, and very difficult to get. We made one of our original Acies in ZDP-189, we have been unable to procure any since. Spyderco and William Henry have also used it on a number of knives.