sedimentary/igneous
Banded Chalcedony (Agate) in Basalt Matrix
Cryptocrystalline Quartz (SiO2) in Mafic Igneous Rock
Hardness: 6.5-7 (Mohs scale), Color: White to grey chalcedony against dark brown/black matrix, Luster: Vitreous to waxy, Crystal Structure: Trigonal (microcrystalline), Cleavage: None/Conchoidal fracture.
- Hardness
- 6
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Physical properties
Hardness: 6.5-7 (Mohs scale), Color: White to grey chalcedony against dark brown/black matrix, Luster: Vitreous to waxy, Crystal Structure: Trigonal (microcrystalline), Cleavage: None/Conchoidal fracture.
Formation & geological history
Formed when silica-rich fluids filled gas vesicles (amygdules) within cooling basaltic lava flows. Over time, the silica crystallized into chalcedony/agate. Often found in volcanic terrains of various geological ages from Precambrian to Cenozoic.
Uses & applications
Commonly used for lapidary work (cabochons), jewelry, and as decorative pocket stones or metaphysical 'worry stones'.
Geological facts
Agates are essentially fossils of ancient gas bubbles in volcanic rock. The micro-bands seen in the white areas are caused by periodic changes in the chemical concentration of the silica fluids during deposition.
Field identification & locations
Identify in the field by looking for smooth, waxy patches or 'eyes' embedded in a rougher, darker igneous matrix. They are most commonly found in gravel pits, riverbeds, and along volcanic shorelines like the Lake Superior region.
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