n the spring of 2000, I was considering an offer to revise one of the Texas collecting guides. To familiarize myself with them, I reread many of the older guides to see what kind of a task I had. The one thing that impressed me was that petrified wood was found in all parts of the state. That is not surprising because most of Texas is composed of sedimentary rocks, and many formations have layers of volcanic ash (tuff) mixed in with them—a combination that can be very conducive to the formation of petrified wood. The book was concerned only with the petrified wood that was suitable as a lapidary material; however, petrified wood is just a generic term and literally means, “wood that has turned to stone.” One geological dictionary (AGI1957) says that petrified wood and silicified wood are the same thing (which in fact they are), but petrified wood includes many more types of wood replacement, none of which are silicified, and most are not good lapidary material.

This past summer Chris Peek left some specimens in the library that he collected at a strip mine near Jewett in Leon County, Texas. It contains smoky quartz crystals up to 2 mm across lining elongate cavities in a soft carbonitized wood that definitely is not a lapidary material but is quite interesting. Similar quartz crystals occur in silicified wood obtained several years ago by Rob Lavinky from near Mount Pleasant in Titus County. Scott Singleton says that both are the same age (Eocene) and from the Wilcox Formation, but neither is listed in the collecting guides because they do not make good lapidary material. Another interesting locality for carbonitized wood is the Alvoro pyrite locality (Mitchell 1987) in Wise County where logs of carbonitized wood are abundant in the Lower Cretaceous age Trinity sandstones. If you think that this mention of petrified wood means lapidary material, you would be very wrong.

Opalized wood is another form of petrified wood common in some areas of the central Gulf Coast. It can make interesting lapidary material but is much softer than the silicified wood and so has a more limited use as a lapidary material.

Chalcocite, a gray copper sulfide, replaces wood in the Permian red beds of Archer, Baylor, Foard, Hardeman, Haskel, Jones, King, Stonewall, and other West Texas counties. Most of this “petrified wood” is small pieces, and the replacement by chalcocite is complete.

I have a small piece of barite from the Big Spring area of Howard County that is supposed to have replaced wood. I have yet to verify this from the literature or by my own examination of the specimen, but I have no reason to doubt it.

Goethite (limonite) is a hydrous iron oxide that along with siderite is an ore for iron in East Texas. The goethite occurs as stalagtites, concretions, box-work masses, and is reported as a replacement of wood in Marion County (Eckel 1938). It probably also occurs as such in surrounding counties, but the replacement may be so crude that it is not recognized as petrified wood.

Uranium minerals have an affinity for carbonitized wood and may partly or completely replace it. Some of the Gulf Coast uranium mines have much of their ore in carbonitized wood or plant remains, but almost no complete replacements like those originally mined in the Colorado Plateau have been reported.

Silicified petrified wood is the most common replacement in Texas and elsewhere in the western United States. The silica may replace and preserve the actual wood cells. Some silicified wood may show bands, lines, and feathers and be considered agatized, but the composition is the same. No matter if the wood is a white chalcedony or a black flint, the composition is basically the same. It only takes trace amounts of iron, manganese, or other minerals to color it, and they are generally not enough to change the over-all chemical composition.

Just remember that when you say “petrified wood,” it is not all the same composition and perhaps you should be more specific. Why did I not take on the revision of the Texas collecting guide? Too much work required in too short a time with too little reward.

References:

American Geological Institute 1957. Dictionary of Geological Terms. Doubleday, NY.

Mitchell, J,R. 1987. Gem Trails of Texas. Gem Guides Book Company, Pico River, CA.

Eckel, E. B. 1938. The brown iron ores of eastern Texas. U.S. Geological Survey bulletin 902.