I grew up in the small town of Petoskey, MI. You can indeed just walk down the shores of Lake Michigan and find these. People at local camps come and gather them one day and then spend the rest of their vacation polishing them in rock polishing stations and fashioning them into jewelry and stuff. All the gift shops sell Petoskey stone paper weights, knives, boxes, necklaces, Michigan-shaped Petoskey stones, you name it!