Depends. I have used a gun flint. If not a pro I would use a peice that fits your hand good,the main thing is to get a sharp edge to cut the steel. When you do any knapping always wear safety glasses. I make most of my gun flints as I pick flint by 5gals in gravels here. Its not the best,but it works.If you check the Foxfire book,I think no5or6 it shows a guy knapping flints. Dilly