Concretion
A hard, often rounded lump that grows inside sediment when minerals slowly glue the grains together, and is often mistaken for a meteorite.
A concretion forms underground when water carrying dissolved minerals seeps through sand or mud and leaves a natural cement around a centre, such as a shell or a grain. Over a long time the cemented part becomes harder than the rock around it. When the softer rock wears away, the concretion is left behind as a ball, a disc or a knobbly lump. The cement is often calcite, silica or iron oxides, which can give a rusty brown or black outer shell.
Concretions matter to meteorite hunters because they are among the most common meteorwrongs, rocks that look like meteorites but are not. Their round shapes, dark rinds and surprising weight fool many people. In Utah, iron-oxide concretions nicknamed Moqui marbles weather out of sandstone by the thousand. Even on Mars, NASA's Opportunity rover found small hematite-rich spheres, nicknamed blueberries, that are thought to be concretions.
To tell the difference, look at a broken or naturally worn face with a hand lens. Concretions often show sand grains or onion-like layers inside, while a real meteorite usually has a thin fusion crust over a quite different interior, often with metal flecks.
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Level: Explorer Ages 10 and up