mineral
Polished agate/chalcedony pebble
Microcrystalline quartz (SiO₂), likely brecciated or mossy agate
AI-generated identification · may be incorrectHardness 6.5–7; translucent brown, gray, cream, and reddish colors with waxy to vitreous luster. Quartz is trigonal, has no cleavage, conchoidal fracture, and specific gravity about 2.6; the rounded surface is polished or water-worn.
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Physical properties
Hardness 6.5–7; translucent brown, gray, cream, and reddish colors with waxy to vitreous luster. Quartz is trigonal, has no cleavage, conchoidal fracture, and specific gravity about 2.6; the rounded surface is polished or water-worn.
Formation & geological history
Forms when silica-rich groundwater deposits layered or branching chalcedony in cavities and fractures of volcanic rocks, commonly during the Cenozoic or older. The specimen was subsequently rounded naturally or by tumbling and polished.
Uses & applications
Used for cabochons, beads, ornaments, paperweights, and inexpensive lapidary pieces; also collected as a decorative stone.
Geological facts
Agate is a variety of chalcedony distinguished by microscopic quartz fibers and commonly visible banding, inclusions, or dendritic patterns. Iron and manganese impurities produce its brown, red, and dark coloration.
Field identification & locations
Identify by its glassy translucency, waxy texture, hardness, and conchoidal fracture; it should not scratch easily with a steel knife. Common sources include Brazil, Uruguay, India, Mexico, Madagascar, and U.S. volcanic terrains.
