Petrified Wood is a type of fossilized remains of terrestrial vegetation resulting from trees or woody plants completely fossilized into stone all the way down to it's microscopic level. Minerals (mostly a silicate, i.e. Chalcedony or Quartz) with the woody structure remaining. Other types of fossils are usually made by impressions.
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Due to lack of oxygen when wood becomes buried under sediment (or volcanic ash) Petrified Wood is preserved and the process of petrifaction begins. When water containing minerals flows through sediment it turns those minerals into the cell structure of the wood which stone overtakes the wood as it decays. 
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Greek:"petros" meaning "stone" or "rock"
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Colors and their containing minerals:
    Black ......... Carbon, Manganese Oxides
    Blue ........... Chromium, Cobalt or Copper
    Brown ....... Iron Oxides
    Green ........ Chromium, Cobalt or Copper
    Orange ..... Manganese
    Pink ............ Manganese
    Red ............. Iron Oxides
    Yellow ...... Iron Oxides, Manganese Oxides