https://en.wikipedia.org/wiki/Petoskey_stone
These are very hard if not impossible to tumble polish whilst a Mohs scale of hardness #3, they disintegrate !
I am using sand paper and a high grit grinder ⚠️ wear a mask ,,, I poisoned my lungs 2 years ago and am still sick ❗️
Note : stones on the left are wet to enhance their appearance.
WãshēKōdä.blog 
Those are cool! Fossilized coral; I never knew that!
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Not bad for a stone surfaced by a glacier 415.6 million years ago
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It’s had a lot of experiences!
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I have been sick 2 yrs after inhaling the dust after sanding !
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That’s not good!
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Quite interesting! No more hurting your lungs though!
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My Doctor has me on an inhaler for ‘black lung decease’ I never mined coal,,,just sanded a stone!
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I’ve always lived in Michigan. When I was young, these were fairly easy to find by the Great Lakes. Not so much these days.
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