sedimentary

Polished black septarian nodule (likely septarian agate)

Septarian concretion composed mainly of microcrystalline quartz/chalcedony, dark calcite or aragonite, and brown iron-oxide mineralization

AI-generated identification · may be incorrect

Hardness is approximately 6–7 for the dark silica matrix, with softer carbonate veins around Mohs 3–4. It is glossy and opaque to slightly translucent, with rounded polished form, dark brown-black color, and contrasting tan-gold vein networks.

Identified August 14, 2026More sedimentary →

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At a glance

Common name: Polished black septarian nodule (likely septarian agate)
Scientific name: Septarian concretion composed mainly of microcrystalline quartz/chalcedony, dark calcite or aragonite, and brown iron-oxide mineralization
Category: sedimentary
Identified: August 14, 2026
Method: Standard

AI Estimated Value

About $10–$40 for a small polished specimen; $50–$150 for larger, well-patterned examples.

Not an official valuation

AI estimate for general interest only — not an appraisal. Actual value depends on condition, provenance, and market demand. Verify with a professional before buying, selling, or insuring.

This identification was generated by AI and may contain errors. Verify important details before relying on them — and never use an AI result to judge whether a specimen is safe to handle. Read the full disclaimer

Physical properties

Formation & geological history

Septarian nodules formed when mineral-rich mud concretions cracked during burial and later filled with silica, carbonate, and iron oxides. They commonly formed in marine sedimentary environments during the Mesozoic–Cenozoic, though exact age depends on locality.

Uses & applications

Used as decorative polished stones, cabochons, paperweights, and collector specimens; exceptionally attractive pieces may be used in jewelry.

Geological facts

The characteristic vein pattern records shrinkage cracks in the original mud nodule. The golden-brown lines are likely iron-stained carbonate or silica veins rather than metallic gold.

Field identification & locations

Identify it by its rounded concretion shape, black-brown matrix, and irregular tan vein network; a steel file should scratch carbonate veins more easily than the silica matrix. Common sources include Madagascar, Utah, Morocco, and Australia.