mineral
Banded agate (likely chalcedony nodule)
Microcrystalline quartz, SiO₂; fibrous chalcedony with possible quartz bands
AI-generated identification · may be incorrectHardness 6.5–7 Mohs; white, gray, cream, and tan concentric or irregular banding; waxy to dull luster; cryptocrystalline structure with no visible cleavage; specific gravity about 2.60–2.65. The rough, rounded form suggests a weathered nodule rather than a cut specimen.
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Physical properties
Hardness 6.5–7 Mohs; white, gray, cream, and tan concentric or irregular banding; waxy to dull luster; cryptocrystalline structure with no visible cleavage; specific gravity about 2.60–2.65. The rough, rounded form suggests a weathered nodule rather than a cut specimen.
Formation & geological history
Forms when silica-rich groundwater fills cavities, vesicles, or fractures in volcanic rock, depositing chalcedony in successive layers. Individual bands may form over long intervals, commonly during the Cenozoic but potentially much older depending on the host rock.
Uses & applications
Used for cabochons, beads, carvings, bookends, and ornamental slabs; attractive pieces have modest lapidary value.
Geological facts
Agate banding records changing chemistry and silica deposition in the original cavity. The specimen may also contain opaque quartz, iron-oxide staining, or small patches of crystalline quartz.
Field identification & locations
Identify by its curved banding, waxy texture, translucent edges, and resistance to scratching by steel; a glassy streak or conchoidal fracture supports quartz. Common worldwide, especially Brazil, Uruguay, Mexico, India, Madagascar, and the western United States.
