Montana Moss Agate

Physical Properties

Hardness: 6.5-7 (Mohs). Color: Translucent gray-to-white background with black/brown dendritic or moss-like inclusions. Luster: Waxy to vitreous. Crystal structure: Cryptocrystalline. Cleavage: None (conchoidal fracture). Specific Gravity: 2.58-2.64.

Formation & Geological History

Formed in the Yellowstone River Basin during the Pleistocene era. These agates formed as silica-rich groundwater filled cavities in basaltic or volcanic rock, where iron and manganese oxides seeped in to create 'moss' patterns.

Uses & Applications

Primarily used in jewelry (cabochons, pendants), lapidary art, and as a popular collector's gemstone due to its unique patterns.

Geological Facts

Montana Moss Agate is found specifically in the gravel beds of the Yellowstone River. The black 'trees' or 'moss' inside are not fossilized plants, but rather manganese and iron oxide minerals like pyrolusite.

Field Identification & Locations

Identify in the field by looking for a 'rough' waxy exterior that appears translucent when held to light. Look for 'river-worn' rounded stones in gravel bars that show a hint of white or gray through the skin.

Identified on: 4/19/2026

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