Not all sand is made of tiny bits of rock. Biogenic sand, which forms from the remains of marine life, is the major ingredient of many tropical beaches. The grains here are tiny fragments of a baby sea urchin shell.

The raised bumps on the white grain represent the sites of insertion for the sea urchin's spines. The blue grain has eroded to the point that the raised bumps have been completely rubbed off. (Magnification 100x)