Compare the disc from Stensö to these "fairy stone" calcium carbonate concretions from the Harricana River in the Abitibi-Témiscamingue admin region, Quebec, Canada. The Stensö piece has been sanded smooth and flat on one side. Locally these things are known as marlekor or mallrikor.
Thanks to project advisor Lotta Feldt of the Östergötland County Museum for setting me on the right track.
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Last year I headed four weeks of excavations at two previously unexplored castle ruin sites near the Swedish city of Norrköping: Landsjö in Kimstad and Stensö in Östra Husby. Finds and written sources suggest that both were built and inhabited in the 13th and 14th centuries.
Myself, Ethan Aines and Mats G. Eriksson are proud to present our report on last year’s fieldwork at Stensö Castle, Östra Husby parish, Östergötland. Lots of goodies there, and with an added meaty report on the bones by Rudolf Gustavsson!
Cool, there are similar rocks that wash out of a bluff near my folks' house, though much darker gray. They fracture in neat, thin layers sharp-edged layers much like the rocks used at Skara Brae, but a short while in the waves weathers them into the most beautiful shapes. I don't remember seeing concentric rings like your find, but they often have holes bored right through them, looking for all the world like an artifact.
Mallrikar, I've seen them in Svensksundsviken not far from the site :-)
A question if you may help me, where can I find more about Herr Holmger and Sigrid? I'd like to read all there is, you see. Thank's in advance
Fia, check out Äldre svenska frälsesläkter. Also Diplomatarium Suecanum 6108 and 6109. Sadly it doesn't seem to be available on-line.