mineral
Brown botryoidal chalcedony (likely iron-stained agate)
Microcrystalline quartz, SiO₂; chalcedony/agate variety
AI-generated identification · may be incorrectHardness about 6.5–7 Mohs; translucent to opaque brown-orange with a waxy to slightly vitreous luster. The rounded, mammillated surface suggests botryoidal growth; individual quartz crystals are microscopic, with no obvious cleavage and conchoidal fracture.
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Physical properties
Hardness about 6.5–7 Mohs; translucent to opaque brown-orange with a waxy to slightly vitreous luster. The rounded, mammillated surface suggests botryoidal growth; individual quartz crystals are microscopic, with no obvious cleavage and conchoidal fracture.
Formation & geological history
Forms when silica-rich groundwater fills cavities and vesicles in volcanic rocks, depositing successive layers of chalcedony; iron oxides commonly produce the reddish-brown color. Exact age cannot be determined from the photograph and depends on its host rock.
Uses & applications
Used as a lapidary material for cabochons, carvings, beads, and polished specimens; tougher pieces may be used ornamentally. Common brown material generally has modest commercial value.
Geological facts
The surface texture is characteristic of botryoidal chalcedony, although a close look or testing is needed to distinguish it from jasper, iron-rich quartz, or fossil material.
Field identification & locations
Commonly found in volcanic terrains, especially Brazil, Uruguay, India, Mexico, and the western United States. Field tests include a glass-scratch test, conchoidal fracture, waxy luster, and lack of fizz in dilute acid; avoid destructive testing on valuable pieces.
