mineral

Polished amethyst-bearing agate geode slices

Microcrystalline quartz (chalcedony, SiO₂) with amethyst quartz and likely iron-oxide inclusions

AI-generated identification · may be incorrect

Hardness 6.5–7 Mohs; translucent gray, cream, and brown banding with purple quartz. Vitreous to waxy luster, trigonal quartz structure, no cleavage, conchoidal fracture; specific gravity about 2.6.

Identified September 20, 2026More mineral →

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Physical properties

Hardness 6.5–7 Mohs; translucent gray, cream, and brown banding with purple quartz. Vitreous to waxy luster, trigonal quartz structure, no cleavage, conchoidal fracture; specific gravity about 2.6.

Formation & geological history

Silica-rich groundwater filled cavities in volcanic rock, depositing chalcedony and later quartz crystals; iron produced the brown coloration. Individual geodes commonly formed in Cenozoic basaltic volcanic environments, though age varies by locality.

Uses & applications

Used as polished display specimens, cabochons, jewelry, and lapidary slices; generally decorative rather than structural.

Geological facts

The concentric bands record repeated changes in groundwater chemistry. The dark central material may be druzy quartz, amethyst, or iron/manganese oxide rather than a separate mineral.

Field identification & locations

Identify by curved agate banding, quartz-like hardness, and crystalline or sparkly cavities; common sources include Brazil, Uruguay, Mexico, and the western United States. Avoid confusing dyed agate with naturally colored material.