mineral
Banded Agate (Polished Slice)
Chalcedony (SiO2) with Quartz
Hardness: 6.5-7 Mohs; Color: Brown, tan, cream, and white banding; Luster: Waxy to vitreous when polished; Structure: Cryptocrystalline; Cleavage: None (Conchoidal fracture)
- Hardness
- 6
- Color
- Brown, tan, cream, and white banding
- Luster
- Waxy to vitreous when polished
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Physical properties
Hardness: 6.5-7 Mohs; Color: Brown, tan, cream, and white banding; Luster: Waxy to vitreous when polished; Structure: Cryptocrystalline; Cleavage: None (Conchoidal fracture)
Formation & geological history
Formed through the deposition of silica from groundwater in the cavities of igneous rocks, primarily volcanic lavas. The banding occurs due to rhythmic crystallization or changes in the chemical composition of the silica-rich fluids over time.
Uses & applications
Used widely in ornamental carvings, jewelry (cabochons), interior decor (coasters, bookends), and historically for precision instruments like mortars and pestles.
Geological facts
Agate is a variety of chalcedony, which is a microcrystalline form of quartz. The distinctive banding is its most identifying feature, and individual patterns are unique to each specimen. It was named by Theophrastus after the Achates River in Sicily.
Field identification & locations
Identify by its translucent nature when held to light and its characteristic concentric or planar banding. Commonly found in Brazil, Uruguay, and various locations across the United States. Collectors look for sharp contrast in bands and lack of fractures.
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