Auction Win: Magnetite and Dolomite

Locality: Fresno County, California, USA

Size: Thumbnail - 1.7 × 2 × 1.5 cm

Magnetite is an iron oxide, Fe3O4, and dolomite is a calcium-magnesium carbonate, CaMg(CO3)2. Magnetite was first described in 1845 by Wilhelm von Haidinger, and its name comes from Magnesia, Greece; dolomite was named in 1791 by Déodat de Dolomieu for the French geologist who studied the Alps. Magnetite is a major iron ore, while dolomite is a common rock-forming mineral in limestone and marble.

Fresno County in central California sits in the Central Valley and extends eastward to the foothills of the Sierra Nevada, with the Fresno area occupying part of the east-central San Joaquin Valley. It is a broad mineral district setting associated with Sierra Nevada and Foothill belt geology, and collector-locality pages for the county highlight numerous mines and districts across the region. Notable documented localities include the Fresno Mine in the Foothill Copper Belt and the Big Creek and Dinkey Creek mining areas in the Sierra Nevada, reflecting the county’s long history of producing specimen-bearing mining camps. For collectors today, Fresno County remains relevant as a large and geologically diverse source area with multiple documented mineral localities rather than a single famed mine.