I'm guessing it's tool steel left full hard and not tempered. I have one from a different source in Canada. Well worth the money, but if you're careless with the file, you can cut grooves in them. I'm planning on saving up my coins for one of the carbide file guides. At any rate, it sure beats trying to file nice square shoulders by eye! I'm guessing one of the more experienced makers will have an opinion or two.