Petrified wood, (from the Greek root petro meaning "stone." In the simplest of terms, the wood turns into stone. The organic materials have been replaced with minerals. Unlike other fossils which are typically impressions, petrified wood is a three-dimensional. This takes place underground, when wood becomes buried and is preserved due to a lack of oxygen. Mineral-laden water flowing over the tree places minerals in the plant's cells thus creating decay and converting the wood to stone.