Cuttlebone's microstructure sits at a 'sweet spot'

A professor has a lesson in one of his mechanical engineering courses on how brittle materials like calcium carbonate behave under stress. In it, he takes a piece of chalk composed of the compound and snaps it in half to show his students the edge of one of the broken pieces. The break is blunt and straight.

source https://www.sciencedaily.com/releases/2020/09/200911141711.htm