What could this be?

[Jordan62]

It's black on the inside, magnetic, not volcanic. We also found more pieces like it

 

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[StoneWhisper]

Perhaps a foselized meadow muffin..

 

[Bajahunter]

Why do you think it is not volcanic?
I can't tell from the pic but it looks possible.
Magnetic? It will attract a paperclip? Or just metalic, iron bearing?