They're made from the leaf springs of trucks, which is 5160 carbon steel, a super tough alloy.

If you go to the HI site, they have pics of how they're made. They heat them in a charcoal grill, forge them with a hammer on an improvised anvil, perform a differential heat treat using a bucket of charcoal and water poured by hand from an old tea pot, then do the final grind on an old grinding wheel that is attached to an old bicycle mechanism where one guy pedals and the other guy grinds. The mirror polish is achieved by putting some finely powdered natural stone in a piece of cloth and rubbing by hand. Simply amazing and tough as nails. The ang khola models are 3/8-1/2" thick, convexed, and virtually indestructible.