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heavy round rock found on beach in north west washington state

I found this 'rock' on the beach in North West Washington State.

It's almost perfectly round save for a few bumps, it's dark grey in colour, solid, feels smooth like any stone you'd find on the beach, and a little grainy in some places.
It's not metallic or magnetic, it's about a 6 on the hardness scale, and it's very heavy.
It seems to have 'seams' like natural cracks, but it's not frail.

Nothing chips off.

I just have no clue, and I'm worried that cutting it open to see what's inside might ruin whatever it is.

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  • $\begingroup$ I don't know why this question got close votes, it has plenty of information. The only thing that was hidden in the image link was the location. I have edited the question. $\endgroup$ – Jan Doggen May 10 '19 at 18:40
  • $\begingroup$ Some kind of garnet crystal? If hardness > 6. $\endgroup$ – Lehs May 11 '19 at 11:22
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What you have found is most likely a milling ball,You can probably find more of the same in the area where you found it.

Multiple balls are put in a rotating horizontal drum and the material you want to crush is slowly added to the drum.

https://en.wikipedia.org/wiki/Ball_mill

The one you have found is probably made of granite but steel balls are more common now in modern mills.

I am not sure of what material your ball is made of so it can be any type of rock or it might be a man made material like ceramic.

Close to where i live was a cement factory and they used steel balls in the production,When the balls was worn out they where thrown out(they became excellent slingshot ammo).

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