PETRIFIED WOOD IS ANCIENT FOSSIL WOOD THAT HAS BECOME SEDIMENTARY ROCK (SOMETIMES WITH IGNEOUS INTRUSIONS OR PARTS), BUT WHICH STILL CLOSELY RESEMBLES WOOD IN STRUCTURE OR COLOR.  HERE IS A DESCRIPTION OF THE PROCESS BY WHICH WOOD BECOMES PETRIFIED, FROM ONE OF THE BEST-KNOWN PETRIFIED WOOD SITES, PETRIFIED FOREST NATIONAL PARK IN ARIZONA:

"Some logs were buried by sediment before they could decompose while volcanoes to the west spewed tons of ash into the atmosphere. Winds carried ash into the area where it was incorporated into the thickening layers of sediment. Ground water dissolved silica from the volcanic ash and carried it through the logs. This solution filled the cells and sometimes replaced the cell walls, crystallizing as the mineral quartz. The process was sometimes so exact the resulting fossils show many details of the logs’ original surfaces and, occasionally, the internal cell structures. Iron and other minerals combined with quartz during the petrification process, creating the brilliant rainbow of colors. Sometimes crushing or decay left cracks in the logs. Here the growth of quartz crystals was not limited and larger crystals of clear quartz, purple amethyst, yellow citrine, and smoky quartz formed."

ACCORDING TO THE AUDUBON BOOK BELOW, THE IRON AND OTHER MINERALS OR ELEMENTS (SUCH AS COPPER OR MANGANESE) ARE PRESENT AS IMPURITIES IN THE WOOD.  THESE OTHER MINERALS CAUSE THE DAZZLING COLORS PRESENT IN THESE FOSSILS.  THE PROCESS OF FORMING THIS WOOD REQUIRES SEVERAL MILLION YEARS.  

SOURCES:

MANNING, REG.  WHAT IS ARIZONA REALLY LIKE?  PHOENIX:  REGANSON CARTOON BOOKS, 1992.  (THIS BOOK OFFERS A LIGHT-HEARTED LOOK AT THE PETRIFIED WOOD PHENOMENON.)

NATIONAL AUDUBON SOCIETY FIELD GUIDE TO THE SOUTHWESTERN STATES.  NEW YORK:  CHANTICLEER BOOKS/ALFRED A. KNOPF, 1999, P. 29.  

PETRIFIED FOREST NATIONAL PARK WEB SITE (WWW.NPS.GOV/PEFO).