Mineral (variety of Chalcedony/Quartz)
Banded Agate (specifically Botryoidal/Orbicular Agate)
Silicon Dioxide (SiO2)
Hardness: 6.5-7 on Mohs scale. Color: Brown, tan, and blue-grey with concentric banding. Luster: Vitreous to waxy. Crystal Structure: Hexagonal (cryptocrystalline). Cleavage: None (conchoidal fracture). Specific Gravity: Approximately 2.6.
- Hardness
- 6
- Color
- Brown, tan, and blue-grey with concentric banding
- Luster
- Vitreous to waxy
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Physical properties
Hardness: 6.5-7 on Mohs scale. Color: Brown, tan, and blue-grey with concentric banding. Luster: Vitreous to waxy. Crystal Structure: Hexagonal (cryptocrystalline). Cleavage: None (conchoidal fracture). Specific Gravity: Approximately 2.6.
Formation & geological history
Formed through the deposition of silica-rich groundwater within cavities (vesicles) of volcanic igneous rocks or sedimentary voids. The banding is created by successive layers of silica that precipitate over time. Geological age varies greatly by location, from recent to hundreds of millions of years old.
Uses & applications
Primarily used in jewelry (cabochons), interior decoration, lapidary arts, and gemstone collecting. Historically used for making mortars and pestles due to its hardness.
Geological facts
The banding in agate is often so fine that a single inch can contain several thousand individual layers. It has been used as a decorative stone since Ancient Greek times, notably for seal stones and amulets.
Field identification & locations
Identify in the field by its waxy luster, transparency when held to light (translucent), and characteristic concentric banding or 'eye' patterns. Common locations include Brazil, Uruguay, and the Lake Superior region of the USA. Many collectors find them on beaches or in dry riverbeds.
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