STUDYING THE PAST OF Petrified Wood
Rock&Gem Magazine|July 2021
Trust Plant Anatomy To Be Your Guide When Working In the Present
DOUG FOSTER
Petrified wood is one of the least expensive semi-precious gemstones and yet can be one of the most fascinating if properly prepared and labelled. Many years ago, I got interested when a big flood dropped tons of rock on our pasture, including a lot of petrified wood, and a local rock shop cut a big chunk for me that revealed unexpected patterns inside. Over time, this interest turned into an obsession. Lacking any formal background in geology and botany, I studied diligently, learned to identify wood, kept my three slab saws running most daylight hours, and identified thousands of specimens of petrified wood. For the last 20 years, I’ve been the curator of petrified wood at the Crater Rock Museum in Central Point, Oregon, and I’ve coauthored and published the first scientific study of petrified wood in Oregon’s Rogue Valley.

To know how best to cut petrified wood, you’ll have to learn some plant anatomy, and to describe and label petrified wood, you’ll need to learn more plant anatomy and buy a 20X lens: your old 10X loop isn’t powerful enough. One attraction of petrified wood, seldom taken advantage of by lapidaries, is telling its story: petrified wood was once alive, and most people are interested in fossils. This can vary from describing woody features to telling how long ago the tree was alive to identifying what kind of tree it was.

CUTTING, GRINDING AND GLUING

When cutting petrified wood, take a hint from how sawmills cut logs and vary the angle of cut to fit the variety of tree being cut. Most logs are “board-cut,” a cut made parallel to the length of a log that doesn’t go through the pith in the center (a/k/a “tangential cut,” per wood scientists). Board-cuts show annual growth rings as repetitive, wavy, oval or “V” or “U” shaped patterns, as can be seen on plywood and dimensional lumber. These patterns will differ, of course, based on whether the board cut is made closer to the outside of a log or closer to its center. Board cuts, however, will create such patterns only if you’re cutting trees that grew in a temperate climate, which have growth rings marking the seasonality of wood growth from spring to the annual dormant winter season.

Tropical trees, in contrast, grow year around and so lack annual growth rings; when board-cut, they won’t show such attractive patterns. In the western U. S., most petrified wood older than the mid-Eocene (about 48 million years ago) grew in the tropics and lacks growth rings. If you collect petrified wood specimens in the field, you can easily learn how long ago those trees were alive by contacting the federal or state land management agency closest to your collecting area, or you can check geologic maps. The very old petrified trees from Arizona and Utah, dating to the Jurassic and late Triassic, lack annual growth rings because they grew in the tropics; they may, though, have a few sporadic rings marking droughts. Much such petrified wood is so colorful it’s called “rainbow wood” and will show bright, attractive colors however it is cut. Other highly colorful varieties of petrified wood, like “Roxy Ann wood” from southwest Oregon, can also make attractive cabs regardless of the angle cut, as shown in the nearby photo.

TEMPERATE CLIMATE TREE PATTERNS

Temperate climate trees can also reveal attractive patterns if “cross-cut,” a cut made perpendicular to the length of the log. This is the cut timber fallers make at the base of a tree trunk to drop the tree (a/k/a “transverse cut”). When a tree with growth rings is cross-cut, its rings show as concentric circles, like a bull’s eye target, and its rays – which carry nutrients from a tree’s outer, living sapwood to its center – show as narrow lines going across, and at right angles to, these rings. In round limbs, rays look like the spokes of a wheel as shown in the nearby drawing but appear to be nearly parallel lines in chunks of petrified wood from big limbs or logs.

While board-cut, temperate climate trees sometimes show growth rings as wide, wavy patterns too large to capture in a small cab. Cross-cuts of such trees will usually capture smaller patterns that both fit within a cab and are often appealing to the naked eye. Before making my first cab, I’d spent 10 years cutting petrified wood specimens, and every one of those cuts was a cross-cut because I was obsessed with identifying wood varieties. To identify wood with a 20X lens, you must study the cross-sectional face of a limb’s vascular system (“plumbing system”), which moves water up from roots to leaves.

Trees with bold growth rings can be showy both when cross-cut and when board-cut. Wood anatomists call these trees “ring porous” because they develop large pores in the spring to carry water from roots to growing leaves, but only small pores in the summer; this contrast prominently marks annual growth rings. Ring porous trees include oak, locusts, ash, sassafras, and elm. In contrast, “diffuse porous” hardwoods – like willow, poplar, cherry, and maple - lack bold growth rings because there is no noticeable difference in pore size throughout the year.

Conifers, which evolved earlier with a less efficient plumbing system, move water from roots to needles through a mass of tiny, straw-like structures called “tracheids.” When viewed with a 20X lens in cross-section, masses of tracheids look like a close-up view through a window screen, and even a sudden change in tracheid size from larger in spring to smaller in summer may not be eye-catching when seen with the naked eye.

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