Wikipedia:POTD column/December 15, 2006

Petrified wood, such as this sample found in Petrified Forest National Park, Arizona, United States, is a fossil wood where all the organic materials have been replaced with minerals, while retaining the original structure of the wood. The petrifaction process occurs underground, when wood becomes buried under sediment. Mineral-rich water flowing through the sediment deposits minerals in the plant's cells and as the lignin and cellulose decay, a stone cast forms in its place.