mineral
Botryoidal chalcedony (likely blue-gray agate/geode material)
Microcrystalline quartz, SiO₂ (chalcedony, locally amethystine quartz)
AI-generated identification · may be incorrectHardness about 6.5–7 Mohs; waxy to vitreous luster, white/gray with lavender-purple and brown iron-oxide staining. Typically cryptocrystalline quartz with botryoidal, porous surfaces and no obvious cleavage; specific gravity about 2.6.
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Physical properties
Hardness about 6.5–7 Mohs; waxy to vitreous luster, white/gray with lavender-purple and brown iron-oxide staining. Typically cryptocrystalline quartz with botryoidal, porous surfaces and no obvious cleavage; specific gravity about 2.6.
Formation & geological history
Forms when silica-rich groundwater fills cavities and vesicles in volcanic rock, depositing concentric chalcedony layers; purple color may reflect trace iron and irradiation. Usually Cenozoic to older volcanic-hosted material, though age cannot be determined from the image.
Uses & applications
Used as a lapidary stone, cabochon, ornamental specimen, and collector mineral; attractive sections may be polished or sold as geode material.
Geological facts
The rounded “grape-like” texture is called botryoidal; cavities can later fill with quartz or amethyst crystals. The specimen may contain mixed chalcedony, quartz, and iron oxides rather than one pure mineral.
Field identification & locations
Test for glassy hardness by whether it scratches ordinary glass, inspect broken surfaces for waxy conchoidal fracture, and check for layered banding or quartz crystals; common in volcanic regions worldwide, especially Brazil, Uruguay, Mexico, and the western United States.
