mineral
Tumbled chalcedony (likely milky agate)
Microcrystalline quartz, SiO₂ (chalcedony)
AI-generated identification · may be incorrectHardness 6.5–7 Mohs; cream to honey-brown, translucent, waxy to vitreous luster, cryptocrystalline structure, and no obvious cleavage. Brown areas likely iron-oxide inclusions or staining; its rounded shape indicates tumbling.
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Physical properties
Hardness 6.5–7 Mohs; cream to honey-brown, translucent, waxy to vitreous luster, cryptocrystalline structure, and no obvious cleavage. Brown areas likely iron-oxide inclusions or staining; its rounded shape indicates tumbling.
Formation & geological history
Forms when silica-rich groundwater fills cavities and fractures, precipitating microscopic quartz fibers, commonly in volcanic rocks. Deposits range from ancient Paleozoic to Cenozoic formations, depending on locality.
Uses & applications
Used for cabochons, beads, carvings, decorative stones, and lapidary practice; durable but generally modestly valued unless strongly banded or exceptionally translucent.
Geological facts
Chalcedony is quartz in a microcrystalline form; “agate” specifically refers to patterned or banded chalcedony. The specimen may also be marketed informally as tumbled quartz or milky agate.
Field identification & locations
Check for glassy/waxy luster, translucency, conchoidal fracture, and inability to scratch it with a steel knife; quartz should scratch window glass. Common sources include Brazil, Uruguay, India, Madagascar, and the western United States.
