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.