Tempered, hard, sharp, steel. This three sided mortis marker is made from tough stuff, made to stand up to the beating that your hammer will give it.
After you have bored the hole into the door jamb, insert the pilot of the mortiser, then strike the end of the shank with a hammer.
It will cut the outline of the strike mortise with its sharp edges. Use your chisel to complete the mortise to the proper depth.