Natural or used for something?

[Ko78]

I'm wondering if anyone can tell me about this piece I found.

 

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[Tdog]

I'd say it's a concretion--a natural formation. Keep looking though, you have the right idea!

 

[The Grim Reaper]

Natural for sure.

 

[Ko78]

I thought as much since it didn't really look like anything hard wore it down. Thanks for the replies, always learning and exploring.