What causes this?

[BearCreek]

We have a creek on our NWGA farm with a lot of shale, slate, sandstone, and chert. I haven't seen this type of stone before. It's not smooth and it feels like there is a big bubble in it. It has all these crazy scallops on the edges and is curved. Anyone know what type of stone it is and what causes it to wear like this? Thank you for your help!

 

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[kcm]

I'm no geologist(!!), but it looks to me like it could be some of that famous Georgia clay that built up in a rock bed, waters dried up or changed course, and then hardened into a stone-like formation. The scallops could be where it was in contact with other river gravels. ...Just guessing here!

 

[Tigger]

Can't see detail enough from pix, but maybe fossilized bone?

 

[HobbyHuntress]

It looks like a piece of concrete that's been mixed with a hint of powder dye. Can you take a pic of the scalloped side? That may be more telling than the current angle.

 

[Eu_citzen]

My hunch tells me it might be man-made.