Petrified wood

Petrified wood is a type of fossil, in which the tissues of a dead plant are replaced with minerals. The petrifaction process occurs underground, when a forest suddenly becomes buried under sediment. Mineral-rich water flowing through the sediment deposits its minerals in the plant's pores and eats away the organic lignin and cellulose until the plant has turned completely to stone. The crystals sometimes reflect the internal strucutre of the plant in which they form, such as tree rings.

A famous petrified wood site is Petrified Forest National Park.