This sort of structure where a hard but brittle material is mated with a softer but elastic substrate is also one of humankind’s great innovations. Through trial and error bladesmiths learned to work steel in a way such that the edge of a tool or weapon blade would harden and better keep its sharpness, while the spine would be made flexible and shatter-resistant. These opposite properties are now known to arise from different crystalline phases, or allotropes, of (carbon-alloyed) iron, in particular martensite and austenite.