Round, unusually smooth rocks turn up often enough in gravel beds and eroding cliffs that they get mistaken for almost everything except what they actually are: dinosaur eggs, meteorites, even fossilized skulls, depending on the shape and whoever’s doing the guessing. Most of the time, they’re concretions, and the real explanation is a slower, stranger process than any of those.
Cementing from the inside out
A concretion forms when mineral-rich groundwater percolating through sediment encounters a small nucleus, a shell fragment, a piece of organic material, even just a grain with a slightly different composition than its surroundings, and begins precipitating cement, often calcite or iron oxide, outward from that central point. Because the cementing mineral grows in all directions at roughly the same rate from a single starting point, concretions tend toward a rounded or oval shape almost by default, hardening into a mass noticeably tougher than the loose sediment around it, which is exactly why they survive erosion that strips away everything else and end up sitting alone on a beach or hillside looking suspiciously deliberate.
Why they get mistaken for other things
A concretion is not itself a fossil, though it sometimes formed around one, and cracking one open occasionally does reveal genuine fossil material at the center that triggered the cementation in the first place, part of why the confusion with fossils in particular is so persistent and not entirely unreasonable. It’s a genuinely different formation story than a transported glacial erratic like petrified wood ending up somewhere unexpected, a concretion forms in place, growing outward from a seed, rather than arriving from somewhere else entirely.
The unsettling regularity of a good concretion, round enough to look manufactured, is really just geometry: mineral cement spreading outward evenly from a point has few other shapes it could reasonably take.
