Except for lin00b's reply, there's not enough emphasis on materials science in this thread.

The harder a material is, the more brittle it is. Once a brittle material has a slight defect, the crack propagates itself. Sure, they still need that certain amount of force to make that crack, but after that it's smooth sailin. What they say, 'a chink in the armour'

For all we know some quacks may have actually make that small notch in the block of materials....

The materials listed that do sound impressive (marble) are classified as brittle, try breaking a piece of ductile metal