mineral

Botryoidal chalcedony with quartz vug

Microcrystalline quartz, SiO₂ (chalcedony), with a drusy quartz-lined cavity

AI-generated identification · may be incorrect

White to pale gray, waxy-to-dull luster with a rough granular vug; hardness about 6.5–7 Mohs, no cleavage, conchoidal fracture, and specific gravity near 2.6. The rounded surface suggests botryoidal growth, while the cavity contains pale quartz or weathered mineral staining.

Identified August 20, 2026More mineral →

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Physical properties

Formation & geological history

Formed when silica-rich groundwater filled cavities or coated surfaces in volcanic rock, commonly during the Cenozoic or older hydrothermal episodes. The hollow formed as gases or soluble minerals were later removed and silica crystallized around it.

Uses & applications

Used as a lapidary and collecting specimen; attractive chalcedony is cut, polished, or made into cabochons and ornamental objects. This small rough piece has little industrial or jewelry value.

Geological facts

Chalcedony is quartz composed of extremely fine intergrowths of quartz and moganite. Botryoidal chalcedony is often called “grape agate” when strongly rounded and purple, though this specimen is not visibly amethystine.

Field identification & locations

Common in volcanic cavities worldwide, including Brazil, Uruguay, Mexico, India, and the western United States. A glassy surface, conchoidal fracture, and ability to scratch glass support quartz; a confirmed identification may require hardness or specific-gravity testing.