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Is this a prairie agate a fossil?

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I found this rock 50 years ago on the farm where I grew up in northern Illinois. The bedrock is far below the surface there. The surface rocks are erratics, dropped by glaciers that moved southwest from Canada through Michigan and into Illinois. It appears to be a kind of agate with some druzy quartz on it. I've recently shown it to several people and have gotten several different opinions on what it is and how it was formed, including that it is some kind of fossil. What do you think?


The surface lines you see in some of the photos (most clearly in the third photo) are narrow and evenly spaced. There are about eight lines per millimeter. I increased the contrast in the ninth photo in an attempt to make the lines more visible. The other photos have not been manipulated, except by cropping.


The pit in the third photo is about 2mm x 3mm and about 15mm deep.










Edited by JimT

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That's a very interesting rock! I think it's safe to say you have a fossil but not sure what of or if it's a combination of fossils. It almost looks like could be algae in some places, or perhaps microbial mat. Basically a mystery to me but others will see this who know more.

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DPS Ammonite

Fossil is likely; what kind is a guess. Consider: stromatoporoid; sponge; coral; bryozoa or stromatolite.


This rock is crystalline and not fibrous or cryptocrystalline like most agates. The areas where the finely spaced layers part and leave domal open spaces suggest the growth patterns of an organism and not that of an agate. (See bottom of third photo for example).

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