This sand slowly dissolves in the rushing stream to give a dilute solution of an acid, silicic acid, H 4 SiO 4 . As the stream slows and eddies, we see a brown scum on the surface of the water. Rocks and plants and twigs are coated with this furry material. These are diatoms, single-celled plants with sturdy cell walls. The cell walls are silica, SiO 2 drawn from the silicic acid in the water and returned to its crystalline precursor.
Diatoms grow in cold water wherever there is light.
They multiply and die. With their death, the silicon dioxide from our chip of granite now becomes a steady rain of microscopic silica dropping to the floor of the sea. It forms deep deposits of the shells of the diatoms. The seas dry up in geological time. And clay deposits bury our diatoms.