Why are blackboards scratched by sand embedded in chalk?

Blackboards are scratched by sand embedded in chalk because the chalk used for writing often contains small particles of harder materials, including sand. When the chalk is dragged across the blackboard surface, these harder particles can scratch and damage the board, creating visible marks and imperfections. Additionally, the abrasive nature of the chalk itself can contribute to wear over time, further degrading the surface. This is why many modern classrooms have transitioned to using dry-erase boards or other writing surfaces that are less prone to damage.