sedimentary
Banded Agate
Cryptocrystalline variety of Quartz (SiO2)
Hardness: 6.5-7 Mohs; Color: Brown, tan, white, and translucent gray; Luster: Waxy to vitreous; Crystal Structure: Trigonal (microcrystalline); Cleavage: None; Fracture: Conchoidal.
- Hardness
- 6
- Color
- Brown, tan, white, and translucent gray
- Luster
- Waxy to vitreous
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Physical properties
Hardness: 6.5-7 Mohs; Color: Brown, tan, white, and translucent gray; Luster: Waxy to vitreous; Crystal Structure: Trigonal (microcrystalline); Cleavage: None; Fracture: Conchoidal.
Formation & geological history
Formed in cavities (vesicles) of volcanic rocks or sedimentary deposits where silica-rich groundwater precipitates layers of microscopic quartz over long periods.
Uses & applications
Commonly used in jewelry as cabochons, for ornamental carvings, polished as pocket stones, and historically for burnishing tools and mortars/pestles.
Geological facts
The banding in agate is caused by the rhythmic precipitation of silica. Every line represents a different pulse of mineral-rich water entering the hollow cavity.
Field identification & locations
Identified by its waxy luster and distinctive concentric or parallel banding. Look for translucent areas when held to a light source. Commonly found in riverbeds and gravel pits.
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