mineral
Tumbled agate or chalcedony pebble
Microcrystalline quartz (SiO₂), likely agate/chalcedony
AI-generated identification · may be incorrectHardness 6.5–7 Mohs; brown, tan, and cream coloration with a waxy to vitreous luster. The rounded shape indicates tumbling; the dark cavity may be a small vug or weathered inclusion. Internal banding is not clearly visible, so precise identification is uncertain.
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Physical properties
Hardness 6.5–7 Mohs; brown, tan, and cream coloration with a waxy to vitreous luster. The rounded shape indicates tumbling; the dark cavity may be a small vug or weathered inclusion. Internal banding is not clearly visible, so precise identification is uncertain.
Formation & geological history
Silica-rich groundwater deposited microscopic quartz fibers in a cavity of volcanic rock, commonly during the Cenozoic or older volcanic episodes. Natural erosion and later polishing produced its present form.
Uses & applications
Used as a lapidary stone for cabochons, carvings, jewelry, and décor; its small size makes it primarily a collectible specimen.
Geological facts
Agate is a variety of chalcedony distinguished by visible concentric or parallel banding, while unbanded material is generally called chalcedony. A rough cavity can expose contrasting silica layers or trapped iron-oxide staining.
Field identification & locations
Commonly found in volcanic terrains, especially Brazil, Uruguay, Mexico, the western United States, and Madagascar. Check for translucent edges, curved fracture, possible banding, and a glass-scratching hardness; a streak and specific-gravity test can help confirm quartz.
