Eric Baccega / Dallol hot spring with salt concretions coloured by sulphur, potassium and iron, Dallol Volcano, Danakil Depression, Ethiopia, March 2015.

Eric Baccega / Dallol hot spring with salt concretions coloured by sulphur, potassium and iron, Dallol Volcano, Danakil Depression, Ethiopia, March 2015.

Jack Dykinga / Concretion balls lining fracture lines in petrified sand dune formation, with lichen scaring calcification marks, Colorado Plateau, Arizona, USA

Jack Dykinga / Eroded striated sandstone with round concretions strewn in geometric patterns, dawn, Vermilion Cliffs National Monument, Colorado Plateau, Arizona, USA

Andy Trowbridge / Long exposure of wave/surf breaking around Moeraki Boulders (60 Million year old mudstone concretions), on Koekohe Beach. Koekohe Beach, Moeraki, Waitaki District, Otago Region, South Island, New Zeal...

Graham Eaton / Cemented nodules within sandstones of the Jurassic, Bencliff grit, Osmington Mills, Dorset, UK. The cementation preferentially develops from a seed grain. The brown staining of the sandstone is oil, a...

John Cancalosi / Septarian concretion or (septarian nodules) from Morocco. Dense mass formed by precipitation of mineral cement in sedimentary rock or soil which is later cracked and filled with other material, in th...

John Cancalosi / Polymetallic nodule, also called manganese nodule, rock concretion formed of concentric layers of iron and manganese hydroxides around a core. From Blake Plateau, Atlantic ocean, collected in 1970's.

Colin Monteath / Moeraki boulders at dawn, an example of septarian concretions, Moeraki Beach, Otago, New Zealand

Colin Monteath / Moeraki boulders at dawn, an example o septariuan concretions, Moeraki Beach, Otago, New Zealand

John Cancalosi / Pyrite on Shale Concretion, Yunnan, China

Tim Fitzharris / Bowling Ball Beach with namesake concretions, Mendecino County, California