A nodule or hard mass of mineral matter that forms inside sedimentary rock. Concretions grow outward from a central core, often a fossil or piece of organic material, as minerals precipitate from groundwater. They are typically harder than the surrounding host rock and often weather out as distinct spherical or ovoid boulders.
Heard on the job
We hit a band of sandstone packed with ironstone concretions that chewed up the drill bits.