sedimentary
Septarian Nodule
Septarian Concretion (Calcite/Aragonite/Barite in Mudstone/Limestone)
Hardness: 3-4 (calcite core) to 6-7 (exterior rock matrix). Color: Tan, beige, brown exterior with deep brown septa cracks and yellow/orange interior minerals. Luster: Earthy exterior, vitreous to resinous along fractures. Texture: Rough, weathered surface with polygonal desiccation-like crack patterns.
- Hardness
- 3-4 (calcite core) to 6-7 (exterior rock matrix)
- Luster
- Earthy exterior, vitreous to resinous along fractures
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Physical properties
Hardness: 3-4 (calcite core) to 6-7 (exterior rock matrix). Color: Tan, beige, brown exterior with deep brown septa cracks and yellow/orange interior minerals. Luster: Earthy exterior, vitreous to resinous along fractures. Texture: Rough, weathered surface with polygonal desiccation-like crack patterns.
Formation & geological history
Formed during the Cretaceous period (about 50 to 70 million years ago) as chemical concretions in marine sediments. Cracking occurred due to dehydration and shrinkage of clay rich mud nodules, followed by mineral precipitation (calcite, aragonite, barite) in the fractures.
Uses & applications
Popular among mineral collectors and lapidaries. Larger specimens are sliced and polished into decorative bookends, display slabs, spheres, and carved ornaments.
Geological facts
The term 'septarian' comes from the Latin word 'septum', meaning partition or wall, referring to the cracks that divide the rock into distinct sections. They are often colloquially referred to as 'dragon stones' or 'turtle stones' due to their cracked exterior shell resembling a tortoise carapace.
Field identification & locations
Identified in the field by its distinct, network-like cracking pattern ('turtle-back' appearance) on a rounded, light brown to gray mudstone nodule. Highly prized when cracked open or cut to reveal calcified interiors. Commonly found in Utah (USA), Madagascar, and England.
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