Iron-rich Siderite Spherule or Concretionary Mass
Inorganic Siderite Concretion (formerly identified as 'Mazon Creek type' nodules) • Pseudo-fossil / Inorganic geologic formation (often mistaken for coprolite or egg)
Geological Period
Likely Carboniferous (Pennsylvanian) to Cretaceous
Estimated Age
Approx. 100 - 310 million years
Preservation Type
Mineralization / Chemical concretion (Iron oxide and carbonate replacement)
Condition Assessment
Fair; heavily weathered surface with significant oxidation but structurally sound
Taxonomic Classification
Non-biological (Sedimentary Mineraloid)
Morphological Description
A dense, rounded nodular morphology featuring an oxidized reddish-brown outer cortex and a darker greyish-black inner core. The surface shows irregular fracturing and a lack of symmetry or internal skeletal structure.
Rock Matrix
Ironstone/Siderite matrix with limonitic weathering (rust-colored staining)
Formation & Location
Commonly found in the Mazon Creek Formation (Illinois, USA) or Bunter Sandstone/Ironstone deposits in Europe
Size & Dimensions
Approx. 4-6 centimeters in diameter; typical for ironstone nodules
Value & Rarity
Estimated Market Value
$5 - $15 USD
Auction Estimate
$10 - $30 USD (only as a novelty or part of a bulk lot)
Rarity Assessment
Common; these ironstone nodules are found in vast quantities in sedimentary deposits worldwide.
Scientific Significance
Provides data on ancient diagenetic processes and the chemical composition of seafloor or swamp-bed sediments; has no biological evolutionary significance unless cracked to reveal a fossil center.
Preparation Recommendations
Mechanical splitting (freeze-thaw cycle or hammer strike) to check for a fossilized core (e.g., fern or jellyfish). Keep dry to prevent further oxidation.
Notable Features
Distinctive 'bullseye' color layering between the oxidized iron-rich exterior and the unweathered interior core.