sedimentary

Septarian Pebble

Septarian nodule / Concretion

Hardness: 3
Color: Tan, brownish-grey with dark brown to black mineral veins
Luster: Dull, earthy matrix with vitreous mineral veining

Identified July 26, 2026More sedimentary →

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Physical properties

Hardness: 3.5 to 7 (varies by mineral composition: calcite, aragonite, siderite, clay/silt material); Color: Tan, brownish-grey with dark brown to black mineral veins; Luster: Dull, earthy matrix with vitreous mineral veining; Structure: Rounded water-worn pebble featuring contraction cracking (septarian structure) filled with mineral precipitates.

Formation & geological history

Formed during the Cretaceous period (about 50 to 70 million years ago) as mud nodules around organic debris on ancient seafloors. Shrinkage cracks formed as the mud dried and compacted, which were later filled by calcite or iron-rich mineral solutions before being eroded into river pebbles.

Uses & applications

Decorative display stones, pocket worry stones, lapidary polishing, educational specimens.

Geological facts

The word septarian comes from the Latin word 'septem', meaning seven, or 'septum', referring to the partition cracks that divide the inner sections of the nodule.

Field identification & locations

Identified by the distinct dark vein-like fractures or rings running through a fine-grained, earthy tan or grey matrix. Often found along stream beds, ocean beaches, or Cretaceous sedimentary formations.