When thinking about opal of any kind, we usually visualize some of the beautiful material photographed for our previous opal articles. But did you know that opal not only forms in the seams, vugs, and cracks of matrix rock but that it can be found as fossils? Let's begin by clarifying what a fossil is. Basically, a fossil is the prehistoric, physical proof of life before recorded human history. Generally, this covers animals and plants that have been "turned to stone" or "petrified," as well traces of early life like footprints and charred wood.
I think the best description comes from 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." For the purposes of this particular article, I will be talking mainly about petrified and "cast" remains.
A "cast" fossil formed when an organic material, such as a tree limb or shell, had been trapped in sediment (sand and clay, or volcanic ash and mud) that dried out and became firm. Eventually, the organic object decayed and dissolved, leaving a cavity or natural mold. Later, rainwater containing materials such as silica filled the mold, and when the liquid evaporated, a "cast" was created. In certain parts of the world, these casts are often found as opal, and can be called "opalized fossils".
Both Australia and Nevada, USA have locations that are well known for their opalized fossils finds. In Coober Pedy, Australia these items are more commonly clam, mussel and snail shells, where silica-rich water seeped into the organism as it was decaying, filling the clam's cavity and replacing the calcium shell as it dissolved. Often part of the original shell is present on the opalized fossil specimen. (Click here
and scroll to the bottom to see an opalized cockle shell.) The fossils that have opalized in Nevada are found as petrified wood casts, some of which are said to show amazing detail of the original wood, in a variety of colors, including the "black" that Virgin Valley is known for. Click here
and scroll down to see an example of opalized petrified wood, captioned "Petrified Wood with Precious Opal," from Virgin Valley, Nevada. Opalized wood can also be found in other North American locations such as New Mexico, Texas, and Arizona.