mineral
Banded agate (likely black/chalcedony agate)
Microcrystalline quartz, SiO₂; chalcedony with thin mineral bands
AI-generated identification · may be incorrectHardness 6.5–7 Mohs; dark gray to black, translucent edges, waxy to vitreous luster, cryptocrystalline quartz with no visible cleavage. The specimen appears naturally fractured or roughly cut, with pale curved banding and possible iron-oxide staining.
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Physical properties
Hardness 6.5–7 Mohs; dark gray to black, translucent edges, waxy to vitreous luster, cryptocrystalline quartz with no visible cleavage. The specimen appears naturally fractured or roughly cut, with pale curved banding and possible iron-oxide staining.
Formation & geological history
Agate forms when silica-rich groundwater repeatedly fills gas cavities in volcanic rock, depositing chalcedony and quartz in layers. Individual specimens may range from Paleozoic to Cenozoic in age, depending on the host lava.
Uses & applications
Used for cabochons, beads, carvings, polished slabs, and ornamental objects; durable rough pieces are also popular among collectors.
Geological facts
The visible curved lines are characteristic rhythmic chalcedony growth bands, though the image alone cannot confirm whether this is agate, onyx, or dyed material.
Field identification & locations
Common sources include Brazil, Uruguay, Mexico, India, Madagascar, and the western United States. Field identification relies on waxy luster, conchoidal fracture, translucency, banding, and resistance to scratching by steel; a quartz scratch test is more reliable.
