In the Harricana River valley in the Abitibi-Témiscamingue administrative region of Quebec, Canada, concretions occur as flat discs, smooth on one side and puffed up on the other with gorgeous patterns. They were formed over thousands of years by deposition of calcium carbonate over small pebbles and fossils. The Indians have called them “Fairy Stones” since hundreds of years, and used to carry them as lucky charms when they went on fishing or hunting expeditions. They believed that wearing a Fairy Stone would protect them against bad spirits and bring them good health and prosperity.
Geologists believe that Fairy Stones may have formed under a glacier, which retreated thousands of years ago. They were then carried by the water and deposited along the shores of certain lakes and rivers. One of the main rivers where Fairy Stones are found is the Harricana River, the second longest river in Canada. It is said that the name "Harricana" came from the Algonquin word Nanikana, which means “the river of biscuits” — biscuits referred to the flat Fairy Stones. A less romanticized version says Nanikana means "the main way".
These stones are often found in soft deposits under clay. The rounded, puffed up shapes come from the growing face that is face down in the clay, while the tops are weathered smooth by the retreating glacier and water. Lying in the mud, they look like normal flat stones, but turn them over and you’ll see many different shapes and formation. Each Fairy Stone is unique.
All photos courtesy of SpiritRock Shop.
This is what you see in the clay.
This is what is looks when turned over.
Sources: Spirit Rock Shop / Wikipedia