What is Peanut Wood?
Peanut wood is a variety of petrified wood that is
usually dark brown to black in color. It is recognized by its white-to-cream-color markings that are ovoid in
shape and about the size of a peanut. It received its name from these peanut-size markings. It is a
fossil gem with a very unusual history.
How Does Peanut Wood Form?
Much of the peanut wood being sold today began its life as a conifer tree on land in the area now known as Western
Australia. When these trees died, rivers carried them into a shallow, salty epicontinental sea that covered much
of what is now the Australian continent.
They arrived at the sea as a piece of driftwood. This was during the
Cretaceous time period, when a species of marine clam
that loved to eat wood lived in the Australian sea. The clam larvae were able to smell nearby wood and swim to
it. When they arrived at a piece of driftwood, they would attach themselves to it and start eating. A tiny pair of valves
soon developed on one end of their long body, and they used the sharp edges of their shell as a rasp. They shaved
off tiny particles of wood - which they would promptly eat. In a few weeks they could excavate a deep tunnel
into the soft, mushy wood.
A few species of these wood-eating clams live in the oceans today. Sailors have cursed about them for hundreds of
years as the enemy of wooden ships. Sailors began calling them “shipworms” because of their long bodies and
their ability to tunnel into a ship much like a worm tunnels through an apple. In the 1700s, shipbuilders began
lining the hulls of their ships with thin sheets of copper to protect
them from the shipworm. Shipworms have been ruining ships, pilings, docks, retaining walls, and other wooden
structures as long as people have been placing them in salt water.
How the Peanuts Form
Back to the Cretaceous seafloor where the waterlogged wood that has been heavily drilled by prehistoric shipworms
is resting. Billions of tiny radiolarians (tiny plankton with siliceous shells) are living in the water above the
wood. A river mouth is a great place for radiolarians to live because the river delivers a continuous supply of
nutrients to the sea. When the radiolarians die, their tiny silicious shells sink to the bottom and accumulate as
a white sediment known as radiolarian ooze.
Layer after layer of radiolarian ooze accumulated over the wood,
entered the bore holes, and some of it dissolved to form a super-saturated silica solution. This dissolved
silica precipitated in the cavities of the wood and replaced the woody tissues, converting the waterlogged wood
into a fossil.
Today, if a piece of the wood is broken, the petrified wood is a brown-to-black color. Contrasting with the
wood is the white radiolarian ooze that filled the boreholes. Since the boreholes are filled, they appear on the
broken surface of the wood as white oval-shaped markings about the size and shape of a peanut. That is how the
peanut wood obtained its distinctive appearance and its name.
Kennedy Range and Windalia Radiolarite!
The sediments that contained the peanut wood lithified into
sedimentary rocks that are now known as the “Windalia
Radiolarite.” The Windalia was eventually uplifted as part of Western Australia’s Kennedy Ranges, which are now above
sea level. A few lapidaries found the peanut wood, tried cutting it and discovered that it is a
gem material that can be used to make
very colorful, interesting, and brightly polished cabochons.
Soon, peanut wood was being used to make clock faces, spheres, beads, and many other lapidary products. Small leftover pieces
from these projects can be loaded into a rock tumbler and used to make
tumbled stones. The gem material is very attractive, and
its unique appearance immediately grabs attention.
Today gem hunters seek out the peanut wood in areas where the Windalia Radiolarite is exposed at Earth’s surface.
It is sold by mail, on websites, in online auctions, and at the Quartzite and
Tucson Mineral Shows where people
from all over the world see it, buy it, and take it home to share with their friends.
It is surprising that an ancient waterlogged wood that was bored by shipworms is now a popular gem that is cut,
worn, displayed, and talked about all over the world.
Contributor: Hobart King
Find it on Geology.com
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|Petrified Wood is a fossil that forms when dissolved material precipitates and replaces wood.
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|Scoria is a dark-colored, extrusive igneous rock with abundant vesicles.
|Three nice peanut wood cabochons cut from material from the Kennedy Ranges of Western Australia. For size reference, the top cab is about 30 millimeters by 20 millimeters in size. These cabs all show boreholes infilled with white radiolarian sediments in brown-to-black petrified woody material.
|A modern clam similar to those that bored the holes in peanut wood. It is called a "shipworm" because it has a long worm-shaped body inside the shell (not visible here). Shipworms still exist and are busy eating any wood that man places in sea water.|
|A close-up of a portion of the slab shown at the top of this page. You can clearly see a number of peanut holes. And, if you look closely you can see a spiral-shaped boring that traverses the width of this image (second row from the bottom).|
|A closer view of one of the cabs in the top image of this column. This cab measures approximately 30 millimeters long and 20 millimeters wide.|