Quartz Geode / Fossil Infill Specimen
Indeterminate mineral replacement / geode cavity infill cf. fossil mold • Mineralized mold/cast or diagenetic mineral formation often associated with fossil cavity infills (invertebrate mold)
Geological Period
Mississippian / Devonian
Estimated Age
320 to 360 million years
Preservation Type
Silicification and secondary permineralization / quartz replacement of an original cavity or fossil mold
Condition Assessment
Good condition; intact outer nodule wall with a partially exposed, sparkling crystalline cavity, minimal weathering damage
Taxonomic Classification
Not directly applicable as this is a mineral formation, possibly replacing an echinoderm or brachiopod mold (Phylum Echinodermata / Brachiopoda)
Morphological Description
Botryoidal to crystalline white quartz/chalcedony lining an interior cavity within a rounded rock nodule, showing microcrystalline sparkle and granular texture.
Rock Matrix
Chalcedonized limestone / dolomite nodule with quartz crystal druse interior
Formation & Location
Keokuk Limestone formation, Midwestern United States (Iowa, Illinois, Missouri region)
Size & Dimensions
Approximately 7 cm to 9 cm in maximum diameter, typical for medium-sized regional geodes
Value & Rarity
Estimated Market Value
15 to 45 USD
Auction Estimate
10 to 30 USD
Rarity Assessment
Common, widely distributed in geode-rich geological formations
Scientific Significance
Illustrates secondary diagenetic processes, groundwater mineral precipitation, and the replacement of organic or void structures within carbonate host rocks
Preparation Recommendations
Clean gently with warm water and mild soap using a soft brush to remove surface dirt; store in a dry environment at room temperature
Notable Features
Contrasting smooth outer nodule surface with a bright white, sparkling crystalline quartz core forming a cavity