sedimentary

Turtle Stone / Septarian Nodule (Water-worn)

Septarian Concretion

Hardness: 3.5 to 4 on the Mohs scale. Color: Brownish with lighter network patterns. Luster: Dull to sub-vitreous. Crystal structure: Amorphous to microcrystalline (composed mainly of calcite, aragornite, and limestone). Specific gravity: 2.6 to 2.7.

Hardness: 3
Color: Brownish with lighter network patterns
Luster: Dull to sub-vitreous

Identified August 1, 2026More sedimentary →

Explore Turtle Stone / Septarian Nodule (Water-worn) in the encyclopedia →

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Physical properties

Formation & geological history

Formed in sedimentary environments, typically from muddy seabeds where volcanic ash or organic matter triggered chemical reactions, creating nodules that cracked internally as they dried and shrank, later filled with minerals like calcite.

Uses & applications

Used primarily as decorative stones, in lapidary arts, jewelry, and by mineral collectors for their unique turtle-back appearance.

Geological facts

The distinctive pattern on septarian stones resembles the shell of a turtle, which is how they earned the folk name 'turtle stones'. The word septarian comes from the Latin word 'septum', meaning partition.

Field identification & locations

Easily identified in the field by its distinctive geometric crack patterns (septae) visible on the surface. Commonly found in locations with ancient marine shale deposits. Collectors look for water-worn pebbles along beaches and riverbeds.

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Fossiliferous Concretion

Septarian Concretion containing Brachiopods/Crinoids

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Banded Siltstone / Mudstone Pebble

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