mineral

Dendritic Agate (Montana Moss Agate)

Silicon Dioxide (SiO2) - Chalcedony with Manganese/Iron inclusions

Hardness: 6.5-7 on Mohs scale; Color: Translucent grayish-blue to chalcedony base with dark brown, black, or metallic dendritic manganese inclusions; Luster: Waxy to vitreous; Crystal Structure: Microcrystalline (trigonal); Cleavage: None, conchoidal fracture; Specific Gravity: 2.58-2.64

Hardness: 6
Luster: Waxy to vitreous

Identified July 26, 2026More mineral →

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Physical properties

Formation & geological history

Formed via secondary chemical precipitation of silica-rich solutions inside rock cavities and fractures. Hydrothermal fluids rich in manganese and iron oxides seeped into micro-fractures in the chalcedony matrix, crystallizing into tree-like or fern-like dendritic patterns.

Uses & applications

Highly valued by rockhounds, lapidaries, and jewelers for creating unique cabochons, pendants, ring stones, and decorative display polished specimens.

Geological facts

Dendritic agate inclusions look like fossilized plants, moss, or trees, but they are purely inorganic mineral growths (manganese/iron oxide crystallizations). Montana Agate is specifically celebrated for its clear-to-smoky translucency contrasted with sharp black dendrites.

Field identification & locations

Identified by its hard, polished quartz surface that cannot be scratched by a steel knife, clear to milky translucency, and distinctive dark branch-like dendritic inclusions. Commonly found along the Yellowstone River basin in Montana, USA, as well as Brazil, India, and Madagascar.

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