Calcite on its own is brittle and cleaves cleanly. The urchin's spine is built from calcite that behaves as a single crystal at the atomic level but is organised into a porous lattice with concentric growth layers and a small fraction of embedded organic material. Cracks that start at the surface run into that structure and are deflected or stopped.
The practical result is a spine that can be blunted and worn without shattering, which suits an animal that lives wedged in surge channels and grinds against rock. The same spines are stiff enough that pieces have been used to write on slate, which is where the common name comes from.
This architecture has been studied as a template for tough ceramics, and the species turns up in materials-science literature more often than most invertebrates. What is still being explored is how the animal controls crystal growth and organic inclusion during spine formation, since the process produces an ordered mineral structure at ambient seawater temperature.