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 plant's own 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.

Petrified wood has a Mohs hardness level of 7.

A famous petrified wood site is Petrified Forest National Park in the U.S. state of Arizona. Wood from the area (though not from the park) is a prized commercial item, particularly cut and polished samples.