Devonian-350 million years. A concretion is a solid mineral in an oval or spherical shape that is formed within layers of sedimentary rock. They are believed to occur when the cementing material precipitates around a nucleus, which might be a leaf, shell or fossil. They range from those needing a magnifying glass to see clearly, up to the 10-foot diameter examples found in North Dakota. This concretion was formed in black shale at the Kettle Point area of the Lake Huron Shore, in Ontario, Canada. Concretions in that area range from a few inches to seven feet across. The Canadian government now protects the remaining concretions.