mineral
Botryoidal chalcedony (possibly quartz geode coating)
Microcrystalline quartz, SiO₂; botryoidal chalcedony
AI-generated identification · may be incorrectHardness about 6.5–7 Mohs; cream to tan, waxy-to-dull luster, translucent edges, and rounded grape-like aggregates. It has no obvious cleavage, conchoidal fracture, and typically a specific gravity near 2.6.
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Physical properties
Hardness about 6.5–7 Mohs; cream to tan, waxy-to-dull luster, translucent edges, and rounded grape-like aggregates. It has no obvious cleavage, conchoidal fracture, and typically a specific gravity near 2.6.
Formation & geological history
Forms when silica-rich groundwater deposits layers of microcrystalline quartz in cavities, commonly in volcanic rock; the specimen appears partly weathered or broken from its host. Such deposits may range from Cenozoic to much older, depending on locality.
Uses & applications
Used as an ornamental and lapidary material, in carvings, cabochons, and mineral collections; less suitable for structural construction.
Geological facts
The rounded “grape” texture is called botryoidal, from Greek meaning grape-like. The cavity and layered coating may indicate a geode or vesicle lining.
Field identification & locations
Identify by its bulbous aggregates, waxy appearance, quartz hardness, and lack of cleavage; a steel knife should not easily scratch it. Common sources include Brazil, Uruguay, Mexico, Morocco, and the western United States.
