Dendritic Agate or Manganese Nodule in Chert/Limestone

Physical Properties

Hardness: 6.5-7 (on Mohs scale); Color: White, cream, or light grey matrix with dark grey to black fern-like or mossy inclusions; Luster: Waxy to vitreous; Crystal structure: Microcrystalline (trigonal); Cleavage: None/Conchoidal fracture.

Formation & Geological History

Formed when manganese-rich water percolates through micro-fractures in volcanic or sedimentary rocks (like limestone or chert), depositing minerals in dendritic (tree-like) patterns as the water evaporates. Usually found in nodules or seams within sedimentary host rocks.

Uses & Applications

Commonly used for lapidary work, jewelry (cabochons), and as a decorative specimen for collectors.

Geological Facts

Although they look like plant fossils, dendrites are purely mineral growths. They are a classic example of 'pseudofossils,' which are inorganic features that mimic the appearance of biological life.

Field Identification & Locations

Identify in the field by its hardness (will scratch glass) and the characteristic 'fractal' or branching dark patterns against a lighter, dense host stone. Found commonly in Brazil, India, and the Western United States.

Identified on: 4/15/2026

Mode: Standard