Quick answer: Concretions are hard, rounded masses that form inside sedimentary rock when minerals such as calcite, iron oxide, or silica precipitate around a nucleus, often a shell, leaf, or bit of organic matter, and cement the surrounding sediment. They are often mistaken for fossils, dinosaur eggs, or meteorites. Famous examples include Utah’s Moqui marbles and New Zealand’s Moeraki Boulders.
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.
Deeper dive: Utah’s Moqui marbles and the “blueberries” on Mars
Moqui marbles are the most famous concretions in America, and the science behind them ended up helping interpret Mars.
- Where: they weather out of the Navajo Sandstone of southern Utah, in places such as Zion, Capitol Reef, Grand Staircase-Escalante, Snow Canyon State Park, and around Moab (SpaceRef, on Chan et al.).
- What: sandstone cemented by iron oxide, from tiny beads to several inches across.
- How: briny, hydrocarbon-rich fluids moving through the sandstone met oxygen-rich groundwater, and iron minerals precipitated where the two mixed. Erosion of the softer surrounding sand later freed the hard balls. They may be around 25 million years old, far younger than the sandstone that holds them.
In January 2004, NASA’s Opportunity rover landed at Meridiani Planum on Mars and found the ground littered with small gray spheres rich in hematite, nicknamed “blueberries.” Within months, on June 17, 2004, Marjorie Chan and colleagues published a Nature paper comparing them to the Utah marbles: both are likely groundwater concretions, which made the Martian spheres evidence that water once flowed through the rock there (SpaceRef).
Can you collect them?
Not in national parks or national monuments, where removing rocks is prohibited. Moqui marbles sold legally come from private land. Ask any seller where theirs came from.
Source: SpaceRef, “Earth Has ‘Blueberries’ Like Mars,” on Chan, Beitler, Parry, Ormo and Komatsu, Nature (2004).
