Beds of chert are commonly associated with volcanic deposits. Diatomite is a white rock composed almost entirely of the siliceous shells of microscopic plants called diatoms. Not all fine-grained siliceous rocks are of organic origin. Some are believed to have precipitated around silica-bearing submarine hot springs. Many have been formed by the replacement of wood, limestone, shale, or other materials by silica-bearing solutions.
Petrified wood is a familiar example.