"Big Red"
By Neal Immega
The story of an exceptional petrified wood specimen at the HMNS
One of the most interesting pieces of petrified wood I have seen anywhere is at the Houston Museum of Natural Science, mounted on the wall under the tail of the Diplodocus. About five feet wide, it is informally known as “Big Red” because of its beautiful coloration. It is amazing that the red, yellow, black, and peach colors are all from iron in impurities in various oxidation states, in different oxides and hydroxides of a range of grain sizes and combinations.  If you were to look at any piece of petrified wood with a 100-power microscope, you would see that most of the color comes from colored pancakes and puffballs floating in a sea of clear chalcedony. The red ones are hematite, the black ones are just really dense red ones, and the yellow – orange – brown ones  are hydrated iron oxides like limonite.
Big Red is an Araucaria species, an ancient type of conifer that can be traced from living trees in Chile to fossils of the Pennsylvanian period, 290+ million years before the present. People who study wood tack the suffix –xylon onto species names to indicate a fossil tree, so the label says Auricarioxylon.  Some related modern trees are the Monkey Puzzle tree and the Norfork Island Pine.
This piece comes from the Chinle formation in Arizona; it’s Triassic in age, about 225 million years ago.  When the army first explored the area, a dry wash that cuts through was named “Lithodendron”, or “Stone Trees”. General Sherman (of Civil War fame) had the army send a couple of logs to the Smithsonian in 1887. About 1890, an industrial facility was built near Holbrook to grind the giant stone logs to make sandpaper. In 1906, President Teddy Roosevelt made a portion of the area a national monument, Petrified Forest, to preserve it for future visitors.
Herbert Zuhl became interested in petrified wood when he stopped at a gravel pit on a ranch outside the park and watched a backhoe load petrified wood on a truck. The rancher offered him logs at the rate of 10 cents a pound. Mr. Zuhl took the risk that he could find a way to cut and polish the logs and that someone would want the results. Mr. Zuhl later donated 200 other specimens to the Museum. Mr. Zuhl uses a sibling slice to Big Red as his office desk! 
Big Red deserves a close-up look. Although the pore space has been filled with silica, the original wood is still present. You can actually dissolve away the silica with hydrofluoric acid and see the cellulose.  All confers have really small wood cells, so that the individual cells do not show up without magnification. The polish on our piece is excellent because it has been thoroughly mineralized. 
  There is something odd about the slab – it is oval and has almost no growth rings in the center area. The center has almost no structure, just like a modern log in the forest that has largely rotted. After our log was buried, it started to petrify in the outer layers before it was crushed by the overburden. The crushing caused the outer layers to break off and get pushed into the center. You can see the compression line across the long axis of the oval where the petrified layers have been broken.
Look carefully and you can see that some of the cavities have been filled with agate.  The whole piece was cracked after petrification and cemented back together with white agate. Mr. Zuhl told me that some of the damage is a lot more recent, though: when the slabs were delivered to his showroom, the movers managed to break the end off each piece. The vertical line a third of the way across from the right is where the slab was glued back together before being repolished. 
 ---adapted from Neal's article in the November 2002 Backbender's Gazette