sedimentary

Boxwork or Septarian Concretion

Septarian nodule / Boxwork weathering

Hardness varies between 3 and 4 on the Mohs scale depending on calcite/limestone content. Colors range from grey, brown, to tan. Dull to subvitreous luster. Monoclinic or trigonal crystal systems for the internal crystalline infill (calcite). Irregular fracture with distinct box-like intersecting mineral veins.

Identified August 5, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed in sedimentary mudstones or shales through the contraction and cracking of a concretion, followed by the precipitation of mineral solutions (often calcite, quartz, or iron oxides) into the resulting fractures over millions of years.

Uses & applications

Mostly prized by rock collectors, lapidary enthusiasts, and for decorative display pieces or polished cabochons.

Geological facts

The intersecting vein patterns create a striking grid-like appearance that often resembles reptile skin or manufactured geometric tiles, leading many people to mistakenly believe they are carved or man-made artifacts.

Field identification & locations

Easily identified in the field by its distinct geometric grid or intersecting raised veins standing out from a weathered host rock matrix. Frequently found in marine sedimentary layers around the world.

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Sandstone or Siltstone

Arenite (if sandstone)

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Fossiliferous Concretion

Septarian Concretion containing Brachiopods/Crinoids

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Quartz pebble

Silicon dioxide (SiO2)

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Generic Pebble / Rock Fragment

Lithic fragment / Weathered rock

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Sedimentary Sandstone

Arenite (Sedimentary Rock)

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Banded Siltstone / Mudstone Pebble

Siltstone / Argillite

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