mineral
Polished brown agate/chalcedony nodule
Microcrystalline quartz (SiO₂), likely brecciated or weathered agate with possible carbonate/iron-oxide inclusions
AI-generated identification · may be incorrectMohs hardness 6.5–7; brown, tan, cream, and rusty-red coloration; waxy to subvitreous luster; cryptocrystalline quartz with no visible cleavage and conchoidal fracture. The porous, pitted areas may indicate vugs or altered host material.
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Physical properties
Mohs hardness 6.5–7; brown, tan, cream, and rusty-red coloration; waxy to subvitreous luster; cryptocrystalline quartz with no visible cleavage and conchoidal fracture. The porous, pitted areas may indicate vugs or altered host material.
Formation & geological history
Forms when silica-rich groundwater fills cavities in volcanic rock, often producing banded chalcedony and agate; iron oxides create the reddish-brown colors. Agates commonly formed during the Cenozoic or older volcanic episodes, but this specimen’s exact age is unknown.
Uses & applications
Used as a lapidary stone for cabochons, carvings, tumbled stones, and decorative objects; generally ornamental rather than industrial.
Geological facts
Its rounded shape and smooth surface indicate natural stream or beach abrasion, possibly followed by light polishing. The irregular internal pattern is typical of an agate or chalcedony nodule rather than a crystalline quartz cluster.
Field identification & locations
Identify by testing hardness against glass, observing waxy chalcedony, and checking for conchoidal fracture; it should not be scratched by a steel knife easily. Common in volcanic regions and gravel deposits worldwide, especially Brazil, Uruguay, Mexico, India, and the western United States.
