sedimentary
Ironstone Concretion
Siderite or Hematite Concretion
Hardness: 5-6 (variable), Color: Rusty brown, ochre, to dark gray, Luster: Dull/Earthy, Crystal Structure: Amorphous or fine-grained crystalline, Cleavage: None (conchoidal fracture), Specific Gravity: 3.0-3.8
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Physical properties
Hardness: 5-6 (variable), Color: Rusty brown, ochre, to dark gray, Luster: Dull/Earthy, Crystal Structure: Amorphous or fine-grained crystalline, Cleavage: None (conchoidal fracture), Specific Gravity: 3.0-3.8
Formation & geological history
Formed by the precipitation of mineral cement in the spaces between particles of sediment. Often forms in marine or lakeside sedimentary rocks when minerals crystallize around a nucleus like a shell or leaf fragment.
Uses & applications
Used historically as a source of iron ore. Today, they are primarily of interest to collectors or used in decorative landscaping.
Geological facts
Concretions are often mistaken for fossils, eggs, or meteorites due to their unusual shapes. Some, known as 'Septarian nodules,' have hollow interiors with beautiful crystal linings.
Field identification & locations
Identify in the field by their rounded or ovate shapes and rusty, weathered exterior. They are commonly found in eroding shale or sandstone cliffs. Collectors look for them to crack open in hopes of finding fossils or crystals inside.
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