Gem Profile- Opalized Fossils

Opalized Fossils

When most people think of opal, they picture the beautiful stones featured in previous articles. But opal doesn't only form in seams, vugs, and cracks it can also be found as fossils.

A fossil is prehistoric physical evidence of life before recorded human history, including animals, plants, petrified objects, and traces like footprints or charred wood.

As described in *The Fossil Book*, fossils are "the remains or traces of organisms that lived during past geologic times and were buried in rocks that accumulated in the earth's outer portion, or crust." This article focuses mainly on petrified and "cast" remains.

A cast fossil forms when organic material, such as a tree limb or shell, is trapped in sediment that hardens. The object decays, leaving a cavity or mold, which later fills with minerals like silica. When the liquid evaporates, a cast is created. In some regions, these casts are made of opal and are called opalized fossils.

Australia and Nevada, USA, are well-known for opalized fossil finds. In Coober Pedy, Australia, opalized fossils are often clam, mussel, or snail shells, where silica-rich water seeped in, replacing the original shell as it decayed. Often, parts of the original shell remain.

Nevada's opalized fossils are mostly petrified wood casts, sometimes showing remarkable detail of the original wood in a variety of colors, including the "black" Virgin Valley specimens. Opalized wood can also be found in New Mexico, Texas, and Arizona.

The definition of a fossil has been debated for years. Lightning Ridge, Australia, claims to have the only "black" fossil opal in the world, so they may not consider Nevada's opalized wood true fossils.

Prehistoric Lightning Ridge was under an inland sea. Today a hot, dry desert, the area yields remarkable fossils, many opalized.

These include dinosaur and shark teeth, various sea shells, pinecones, crocodiles, turtles, marine fishes, and even opalized reptile or dinosaur skin. Imagine uncovering a plesiosaur or dog shark skeleton and finding it opalized!

Many good specimens are damaged during excavation as miners focus on opals with vibrant color, and much of the fossil material is common opal.

The Lightning Ridge region includes opal fields and towns such as Coober Pedy, White Cliffs, Andamooka, Mintabie, and Lambina.

For some stunning examples of opalized fossils from the Australian Museum, please click the following link: opalized dinosaur toe, found in New South Wales, is 8cm (3 inches) long!

Pineapple Opal

There is an extremely rare form of opal known as a "pineapple" that can resemble an opalized fossil pinecone. Actually it is a pseudomorph of glauberite, where the original mineral decayed and was replaced by opal, and it is the most valuable opal in the world! This unusual form of opal is occasionally found in the White Cliffs and Gemville areas of Australia and, believe it or not, this is the most valuable opal formation in the world!

Resources

Print Resources:

Gemstones of the World by Walter Schumann, ISBN 0-8069-3088-8
Minerals of the World by Walter Schumann, ISBN 0-8069-8570-4
Opals by Fred Ward, ISBN 1887651047
The Fossil Book by P.V. Rich, T.H. Rich, M.A. Fenton, and C.L. Fenton, ISBN 0-486-29371-8
The World of Opals by Allan W. Eckert, ISBN 0471133973

Internet Resources:

www.AustralianMuseum.net.au
www.WhiteCliffsOpal.com at http://whitecliffsopal.com/html/introduction.html