sedimentary

Septarian Nodule

Septarian Concretion (CaCO3 with Siderite and Calcite/Aragonite infusions)

Hardness: 3.5-4.0 on Mohs scale. Colors: Brown, reddish-brown, yellow, cream, and white. Luster: Dull/earthy for the matrix, vitreous for the vein fillings. Crystal structure: Trigonal (calcite filling) within a microcrystalline matrix.

Hardness: 3
Luster: Dull/earthy for the matrix, vitreous for the vein fillings

Identified April 11, 2026More sedimentary →

Identify your own rocks.

Get a report just like this from any photo, free.

App Store Google Play

Physical properties

Formation & geological history

Formed during the Cretaceous period (approx. 50-70 million years ago). They began as sedimentary clay balls that shrank and cracked over time, which were then filled with mineral deposits like calcite or aragonite from circulating groundwater.

Uses & applications

Primarily used as a decorative display stone, lapidary material for jewelry (cabochons), and highly prized by mineral collectors for its unique 'turtle skin' geometric patterns.

Geological facts

The word 'Septarian' comes from the Latin word 'septum', meaning partition, referring to the cracks that divide the concretion. These are often found in areas that were once ancient sea beds.

Field identification & locations

Identify in the field by their rounded, potato-like exterior appearance; when cracked open or sliced, they reveal the signature 'lightning bolt' or spiderweb internal vein patterns. Common locations include Utah (USA), Madagascar, and the UK.

Other sedimentary specimens

See all

Sandstone or Siltstone

Arenite (if sandstone)

Sedimentary

Sandstone

Arenite (primarily SiO2)

sedimentary

Shale or Slaty Mudstone

Argillaceous sedimentary rock

sedimentary

Chert Breccia

Brecciated Chert (Microcrystalline Silica)

sedimentary

Sandstone (Quartz-rich)

Arenite (SiO2)

sedimentary

Sandstone (with man-made markings)

Arenite (composed primarily of Quartz, SiO2)

sedimentary