Petrified wood: The name comes from the Greek "petro" or "rock", and refers to wood that has been turned to stone. The process of petrification preserves parts of ancient trees in fossil form. When the wood becomes buried under sediment, mineral-rich water flows through the sediment and replaces the cells' structure with minerals, producing a very good stone copy of the original piece of wood. The predominant minerals in petrified wood are silicates, such as quartz.
It's quite wondrous to examine a piece of petrified wood, knowing that it represents part of a hugh forest existing millions of year ago.