concept

Fracture toughness

A measure of how much stress concentration at a crack tip a material tolerates before the crack runs away catastrophically, written K_Ic and given in MPa·m^0.5. Its foundation is the energy criterion A. A. Griffith published in 1921: a crack propagates once the elastic energy released exceeds the energy needed to create the new surface. Volcanic glass sits at roughly 0.7-0.95 MPa·m^0.5, while hardened steel reaches 25-45, a gap of several tens of times. That single property explains both sides of obsidian: it takes the sharpest edge anyone can knap, and it is the most fragile choice the moment it is bent or struck.