by Mike Fix
Many people, including fossil experts on occasion, have been fooled into thinking they have found a fossil when in fact it is a natural mineral deposit called a nodule or concretion. These geologic curiosities are called "pseudofossils" which means false fossil.
The shape of a nodule or concretion can be highly variable and therefore they can resemble all sorts of fossils including bones, shells, gourds, nuts and dinosaur eggs. Even those of us who have spent years collecting and studying fossils have been momentarily fooled by these things. Sometimes the accidental resemblance to an actual fossil is truly amazing, but the forgery is always revealed upon a closer look. Some pseudofossils have such amazing features that they are highly prized by collectors.
A nodule is a local mineral concentration that is totally different from what the surrounding rock is made of, such as a hard chert (quartz) nodule in a soft limestone (calcite).
A concretion is a local concentration of the mineral cement that binds the particles in certain rocks. For example, a sandstone concretion is just a part of the rock that has more mineral cement than the rest of the rock, and so it is much harder and more resistant to weathering. So, as the softer rock weathers away, the concretion erodes free from the rock.
Probably the most commonly mistaken pseudofossils are chert nodules, which are abundant in many parts of Missouri and Illinois. These lumpy masses of microcrystalline quartz are typically found embedded in, or weathered out of layers of limestone. They are frequently egg-shaped which causes many people to mistake them for dinosaur eggs.
When viewed under a microscope, the surface of a real dinosaur egg shows a complex pattern of tiny, organized structures that varies from place to place on the shell. Whereas a nodule or concretion looks grainy or rocky across the entire surface.
When in doubt about the genuineness of a "fossil" it is always best to check with a paleontologist - a fossil expert. But don't be too disappointed if your dinosaur egg discovery turns out to be a natural forgery.
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