sedimentary
Banded Agate (Quartz)
Agate (Silicon Dioxide, SiO2)
Hardness: 6.5-7 on Mohs scale; Color: Reddish-brown, white, and tan bands; Luster: Waxy to vitreous; Crystal Structure: Cryptocrystalline; Cleavage: None (conchoidal fracture).
- Hardness
- 6
- Color
- Reddish-brown, white, and tan bands
- Luster
- Waxy to vitreous
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Physical properties
Hardness: 6.5-7 on Mohs scale; Color: Reddish-brown, white, and tan bands; Luster: Waxy to vitreous; Crystal Structure: Cryptocrystalline; Cleavage: None (conchoidal fracture).
Formation & geological history
Formed in cavities of volcanic or sedimentary rocks where silica-rich groundwater deposits layers of chalcedony over time. These rounded specimens are often further smoothed by water erosion in rivers or along coastlines.
Uses & applications
Commonly used for ornamental carvings, jewelry (cabochons), tumbled stones for collecting, and industrial mortars and pestles due to its hardness.
Geological facts
Agate is named after the Achates River in Sicily where it was first discovered. The banding is caused by rhythmic crystallization as fluid chemistry changes during formation.
Field identification & locations
Identify in the field by its waxy texture, translucency when held to light, and characteristic concentric or parallel banding. Commonly found in riverbeds, glacial till, and coastal areas.
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