Smooth stone wrapped in rough stone

[Fckwalm]

I’m not sure exactly what this is, but I’ve been going through my grandfathers things and I definitely have a lot of stones which I can identify, but a few I’m not even sure how to look up. I figured someone on here might know right away. So, this is about 11inch diameter, I have not weighed it yet. The grey stone is dimpled but smooth and the outside part is rough and seems to have plant growth in it. What do I have? Thanks for the help!

 

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[Mvgirl]

Looks like mine... found it by the railroad tracks. I accidentally dropped it and wala... a stone was hidden inside!!! Have no idea what type it is though.

 

[DDancer]

Just from the picture the first post looks like a conglomerate, basically a cement of many materials, with a stone trapped inside. The second post is a good example of a concretion. Often concretions can be broken and show layers.

 

[niffler]

The smooth grey part looks like limestone to me.