Wood that is buried in silica-rich sediments is commonly replaced by
quartz, agate, or opal. The wood structure, including a large number of
the annular rings, knots, small branches, and bark, may be preserved.
This process of replacement by silica is believed to take considerable
time. Preservations by other means (_see_ Jet, p. 22) are known, but
silica replacements are most commonly used as gem materials.
Fossil wood is often used by lapidaries as gem material when mineral
replacement preserves the wood structure sufficiently well and when
various impurities color the replacement material attractively.
Excellent gem-quality fossil wood (Pl. I, B) has been found at a great
number of localities in Texas. Agatized and opalized wood occurs in
great abundance along the outcrops of Eocene and Oligocene strata of the
Texas Gulf Coastal Plain. Much of this material is very well suited for
cabochons, bookends, and other lapidary uses. The preservation is
especially good at numerous localities in Washington, Lee, Fayette, and
Gonzales counties, and the variety of colors, such as bluish, gray,
brown, red, yellow, and black, makes this material especially sought
after by “rock-hounds.” Some of the agatized and opalized wood
fluoresces yellow or green under ultra-violet light. The fossil wood is
sometimes found as stumps, limb sections, or large trunk fragments, but
the great majority of the gem material is found as small broken
fragments or stream-rolled cobbles.
Fossil palm wood is by far the most sought after variety because this
material displays “eyes” and tube-like structures that yield very
attractive cabochons and cabinet specimens. Texas fossil palm wood is
highly regarded by cutters from all parts of the country, and this
material is thought by many lapidaries to be some of the finest
gem-quality fossil wood in the United States.
Gravel pits and river gravels in Live Oak County have produced very fine
agatized wood. Although the gem material does not seem to be as abundant
in this area as it is in counties to the northeast, the vivid colors and
excellent preservation of the fossil wood in Live Oak County have
attracted collectors from all over the State. The fossil wood usually
occurs as large rounded cobbles in the streams. Much of this material is
quite translucent when cut and contains various shades of brown, orange,
and red.
The gravels of the Rio Grande have produced some fossil wood in addition
to the excellent agate that is also found there. Most of the fossil wood
found in these gravels is very well preserved, but the colors are
commonly dull shades of brown. Occasional fine red and yellow specimens
have been recovered from the Rio Grande gravels, but these are rare.
Good agatized wood has been found in and near Palo Duro Canyon,
Armstrong County, about 50 miles southeast of Amarillo. Large trunk
sections are not uncommon, but most of the material of cutting quality
is obtained from small fragments. The Palo Duro Canyon fossil wood
greatly resembles the famous Arizona Petrified Forest wood but is not
nearly as plentiful. The Palo Duro wood contains yellow, brown, red, and
bluish colors most commonly. Some of the wood-producing area is within
Palo Duro Canyon State Park which is, of course, closed to collecting.
The surrounding area has been worked diligently by local collectors, but
new pieces of wood are exposed after heavy rains.
Webb and Duval counties have also produced some good fossil wood
specimens.