sedimentary/igneous

Banded Chalcedony (Agate) in Basalt Matrix

Cryptocrystalline Quartz (SiO2) in Mafic Igneous Rock

AI-generated identification · may be incorrect

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

Identified July 1, 2026More sedimentary/igneous →

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Physical properties

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.