sedimentary

Dendritic limestone or agate

Manganese-oxide dendrites, commonly pyrolusite (MnO₂), in carbonate rock or chalcedony

AI-generated identification · may be incorrect

Tan-gray, fine-grained polished specimen with black fern-like dendrites; likely Mohs 3–4 if limestone, or 6.5–7 if agate/chalcedony. Dendrites are surface or fracture coatings, not true crystals; luster is matte to slightly waxy.

Identified August 18, 2026More sedimentary →

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Physical properties

Formation & geological history

Manganese-rich groundwater deposited branching oxide patterns along bedding planes, fractures, or porous surfaces during diagenesis or later weathering. The host may be sedimentary limestone, or silica-rich chalcedony of variable Cenozoic to older age.

Uses & applications

Primarily a decorative and lapidary material used for cabochons, slabs, bookends, and display specimens; it has modest jewelry and collector appeal.

Geological facts

The fern patterns are inorganic mineral dendrites and do not represent fossil plants. Pyrolusite and related manganese oxides commonly create black branching forms called pseudofossils.

Field identification & locations

Identify by the sharply branching black patterns and tan host; test carefully with a steel point and dilute acid—limestone scratches easily and may effervesce, whereas chalcedony does not. Common sources include Mexico, Turkey, India, and the western United States.