What Is This??

[WI. Prospector]

While looking for a good place to prospect for gold (ultra-tiny flakes of flour gold) along the shoreline of Lake Michigan in South Eastern Wisconsin, I came across an area where sand dunes were eroding into the lake. A clay layer was exposed with sand on top, so I started poking around.
I came up with this strange egg shaped object that I could feel was heavier than the normal rock along with a flatter counterpart that perfectly seated the first. My first thought was a clump of weld or something like that. When the two pieces are separated you can see a divot in the center which interlocks the egg like piece perfectly. After staring at these pieces for a while I (very unwisely) decided to whack the egg shaped piece with a hammer to see what the inside looked like. The bottom part was already in two pieces.
Well, It (the egg shaped piece) was hollowed in the center with what appears to be pyrite, but that is my guess.
The surrounding material is silver looking and metallic, but is not effected by magnets. (none of these pieces are)
All photos were taken after I whacked the egg shaped piece with the hammer:
I re-assembled the two pieces for the first photo (as I found these pieces originally)
Thank you all for your time and patience.

 

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[fyrffytr1]

Geode?

 

[geo4472]

Pyrite Coprolite

 

[fowledup]

It's a type of Concretion. Which in your case is concretionary ferric oxides (rust) forming around a non concretionary center. Rattle stones are examples of a type of concretion.

 

[BosnMate]

I see some peacock ore, which is copper. The rainbow colors is what I'm talking about. The other doesn't look like pyrite I'm familiar with. If you have some, try a drop of nitric
acid on it and see what happens. Also, what range does it read out on your detector? Does it say hot rock or is it more specific?

 

[WI. Prospector]

Thanks Leave it to me to open a geode with a hammer

 

[WI. Prospector]

I have no nitric acid or metal detector, i just looked over and there it was sticking out of the sand. Thanks for the input, i will work on the acid and getting a detector (planned to anyways) and post an update a.s.a.p.

 

[WI. Prospector]

Pin-Pointer gave no indication of metal at all from the spherical rock pictured above.

 

[dredgernaut]

petrified acorn is my guess

 

[kcm]

petrified acorn is my guess

I was just about to say this. That first pic looks strikingly like an acorn, although a bit large. But, there may easily have been trees with larger acorns hundreds, thousands or more years ago.

As with geodes, I would suspect that it would be possible for an acorn to sit while water drained through it bringing and depositing minerals. This might also explain the peacock-type coloring in the center. I watched a show (many years ago!) that explained this process and they broke open several geodes that were too far gone to be worth anything (all filled up - no hollow inside) and it seems they had similar colorations (meaning brighter and more distinct) in the centers.