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Kirkus
I came across this rock years ago when I was a teen, I can’t remember exactly where I found it or when. Fast forward 20+ years when I was cleaning out my attic found it . It’s about 4. 5” long and has an egg like shape concave in the middle of the rock. I’ve often wondered what it is and if it had a fossil in it at one time or it was made into that shape by hand? Can anyone please tell me what it is. 5D8CA956-82B8-40C9-ACAA-23A914B1720A.jpeg  3999ECE7-DB5B-425A-A8F7-B7F4E2A98D24.jpeg  ACD6F49E-AED4-4BD4-84D7-AC77C88F2DDE.jpeg 
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Dirty Pete
Hi,
There used to be an ovoid shaped concretion in that space which has since fallen out when the host rock was split. Suggest you google 'concretion' for more info.

Pete
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prep01
Hello and welcome to the forum. This is a water worn boulder if what looks like limestone or mudstone which has formed round a 'nucleus' rock / fossil / mineral and ththen been split open and fallen out  or someone has broken it open to see if there is a fossil inside. In the bottom photo there seems to be a few fragments of fossil broken shell.
Colin Huller
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Barrow Museum
This looks like part of an ironstone nodule, which has formed around a core of claystone.  The claystone will have been a hardened mud pellet that had been ripped up by water currents, rolled around and then incorporated as a pebble within a sandy or silty sediment.  We cannot say from your information whether it was a marine or freshwater deposit and these structures occur in both.  They form after burial when iron in groundwater is attracted to the clay and forms a crust around it, cementing the surrounding matrix.  When eroded from its original layer, the nodule has cracked open and the relatively soft clay interior worn away.  Sometimes these clay pellets shrink and rattle around inside the ironstone nodules.  You may find them described as "box stones".
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Dirty Pete
I'd go with BM's explanation but here's an example of what I was referring to..
b25edf03bf10334c.jpg
  Pete.
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fossiladdict
Hi D.P, is that yours or found image..looks like cotham marble?.stromatolite
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Kirkus
Thank you very much for all your comments, it’s nice to have some knowledge about this strange rock.
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Dirty Pete
Hi F.A, sadly not mine, just a googled concretion for illustrative purposes. I do have some nice examples of Cotham Marble however which occur as arched bioherms. Here's a 2' wide one.
IMG_20200401_123635.jpg 
Pete.
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fossiladdict
arrh yes pete i think i've seen that stunner in one of your previous posts??,your certainly on top of your game on working it to something very desirable...ill have to pop this in my to do bucket list..any tips on finding or looking out for a potential piece?
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