Septarian Nodule

A septarian nodule (or septarian concretion) is a rounded sedimentary nodule crossed by a distinctive network of angular cracks filled with crystalline minerals. Polished, it shows golden-yellow calcite, brown aragonite and grey limestone in a pattern that has earned it the nickname 'dragon stone.'

How they form

The story begins with a mud or clay concretion that forms around organic material on an ancient sea floor. As the nodule dried and shrank it cracked internally into wedge-shaped voids (the 'septa,' Latin for partitions). Over time mineral-rich water filled these cracks, depositing yellow calcite and a brown rim of aragonite and cementing the whole into a solid, patterned stone.

Where they are found

Well-known septarian nodules come from Utah and Madagascar, among other sedimentary basins around the world.

For collectors

Bold, high-contrast crack patterns with bright calcite and a clean polish are the most desirable; septarian is cut into spheres, eggs, slabs and bookends.