Polished surface of a petrified tree trunk, close-up. Petrified (fossilised) wood forms over many thousands of years. The petrification process begins when a tree falls into water or mud. The minerals in the wood are gradually replaced by minerals from the water or mud, which solidify into stone, usually a type of quartz. The petrified wood may remain underground, or may eventually be exposed by erosion of the surrounding material. The largest concentration of petrified wood is the Petrified Forest of Lesbos on Lesbos Island, Greece, where trunks up to 22 metres in length can be found. The forest is estimated to be 20 million years old.