I don't remember all the details, but it has to do with grain size, crystalline structure, and the chemistry of the material itself.
My first father in law has his PHD in Geology from UNC. In the finish to his degree was 2 days of oral exams where his professors assembled in a panel to put the final stamp on his education. I remember he said one professor asked him one question. “What is sand?” The answer took him four hours. He talked about tectonic plates, continental drift and tides as part of his answer.
It's interesting to watch them chewing on rocks like it was popcorn. It makes a lot of noise that carries pretty far.