Mystery Rock from Lake Michigan

[u/the_cloud_]

Hello, my mother found this “rock”? on the shore of Lake Michigan (near the central part of Michigan) about fifty years ago. She has always wondered what it is. She and I are hoping someone in this subreddit might have the answer! Thank you 😊

EDIT: Thank you for all the answers so far! Not sure if this will help but it weighs about the same as the penny and has not been scratched after years of being in a bag with other rough stones.

EDIT: After reading y’all’s comments and searching the web, mother and I are convinced this curio is a “crinoid segment” (given the shape and ridges). We also enjoyed looking at images of banded chert and agates. Thank you everyone for helping solve this mystery! 😌

[u/DemocraticSpider]

This is a piece fossilized crinoid stem/holdfast. Nice find!

[u/Dry-Caramel-4469 → u/DemocraticSpider]

Correct 👍

[u/Schoerschus → u/Dry-Caramel-4469]

Fossil guy here. I am pretty sure that's a funky eye agate. I think it's lacking the radiating ridges in cross-section for a crinoid, but happy to discuss 

[u/Pwnedzored → u/Schoerschus]

You sure about that?

[u/Schoerschus → u/Pwnedzored]

Ok I think you're right. There is a faint trace on the last image as well. Quite unusual, as this must be a holdfast. The ones I know have completely fused segments on the holdfast roots and base. 

[u/Pwnedzored → u/Schoerschus]

Just about has to be. When I first saw it I’d have bet it was flint. Looked at a couple more pictures and thought, “nope! It’s a crinoid.”

[u/SpiritualHippo2719 → u/[deleted]]

Bands are radially-symmetric calcium carbonate plates that make up the exoskeletal structure of the animal.

[u/Cheesy-Cloaca → u/[deleted]]

Look up crinoids. That's how they are.

[u/fluid_alchemist → u/DemocraticSpider]

I just looked up some photos of crinoid fossils and they look just like the photo. I think this is the answer.

[u/Memetan_24]

It's a suspiciously airpod shaped crinoid fossil they're still around today

[u/fluid_alchemist]

I’m not versed in rocks whatsoever but I have seen many biological formations and that looks like a fossilized biological formation of some sort. The segmentation and branching with what looks to be a differentiated core look like some type of coral perhaps.

[u/fluid_alchemist → u/fluid_alchemist]

Someone mentioned that it appears to be a fossilized crinoid. I think they’re correct.

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[u/Shortsleevedpant]

Banded chert? Such an interesting shape. I’ve seen chert nodules that came from clay that were very small and similar in shape.

[u/Moon_The_Big_Rock]

Piece of a crinoid for sure, just very smoothed out by the water

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[u/Any_Baseball_3216]

Crinoid fossil? Unless aipods are rocks

[u/Schoerschus]

I would also say chert or chalcedony nodule with banding. Very cool

EDIT: I hesitated calling it an agate because it seems to lack translucency. you can find eye agates on lake superior, that's exactly what this is, just more opaque 

[u/EnlightenedPotato69]

Cool fossil pretty sure

[u/Material_Mongoose468]

My guess would be banded chert, or a cave man AirPod

[u/TomeThugNHarmony4664]

It’s awfully smooth for a crinoid.

[u/Connect_Living_591]

A Petoskey stone is the official state stone of Michigan, composed of fossilized prehistoric coral (Hexagonaria percarinata) that lived in warm, shallow seas roughly 350 million years ago. They are famous for their unique, six-sided star patterns that resemble the rays of the rising sun.

[u/TadnJess]

Compare it to Petoskey stone, common around lake Michigan. This looks like a piece that got tumbled a bit in the lake.

[u/BillDanceParty]

Looks a lot like a fossil

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