Please help ID, what IS this rock??????????

Magnolia, TX(Zone 8b)

Hey, I'm glad I saw this forum. Can anyone help ID this thing? We bought it in a little shop in Texas because it was so unusual. It's definitely rock, it's about 12" across, weighs about 12 lbs. and sticks out in the middle. It has us stumped. It seems to be made up of little tiny rocks, the stuff between the rocks is definitely sedimentary. I assume that means it's not manmade.

At first we thought it might have been like a grinding wheel, but then it would be smooth from all the friction.

Thumbnail by Ladyborg
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