Septarian Nodule

Physical Properties

Hardness: 3.5-4.0 (Mohs scale); Color: Brown, yellow, grey, white; Luster: Vitreous to dull; Crystal structure: Trigonal (Calcite) or Orthorhombic (Aragonite); Specific gravity: 2.7-2.9.

Formation & Geological History

Formed during the Cretaceous period (50-70 million years ago) when volcanic eruptions killed marine life that sank to the ocean floor. Cracks formed as the organic-rich mudballs dried and shrank, which were later filled with mineral deposits like calcite.

Uses & Applications

Used primarily for ornamental purposes, jewelry (cabochons), interior design (bookends, spheres), and valued as a unique specimen by geological collectors.

Geological Facts

Septarian nodules are also known as Dragon Stones due to their cracked, scaly appearance. The name comes from the Latin 'septum' meaning partition, referring to the cracks that divide the concretion.

Field Identification & Locations

Identify by its distinctive pattern of radiating cracks (septaria) filled with yellow calcite and dark siderite/clay. Commonly found in Utah, USA, and Madagascar. Look for round mud-like nodules in shale beds.

Identified on: 5/13/2026

Mode: Standard