mineral

Polished agate fragment

Chalcedony (microcrystalline quartz, SiO₂), likely brecciated or banded agate

AI-generated identification · may be incorrect

Hardness 6.5–7 Mohs; pink, cream, gray, and reddish-brown bands with a waxy to vitreous luster. Quartz is trigonal but occurs here as cryptocrystalline chalcedony; no obvious cleavage, conchoidal fracture, density about 2.6 g/cm³.

Identified August 10, 2026More mineral →

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Physical properties

Formation & geological history

Agate forms when silica-rich groundwater fills cavities in volcanic rock, depositing chalcedony and quartz in successive bands; most examples formed during the Cenozoic or older volcanic episodes. This piece appears cut or tumbled and broken.

Uses & applications

Used for cabochons, beads, carvings, ornaments, and lapidary specimens; not generally used structurally. Value is mainly decorative or educational.

Geological facts

The contrasting bands arise from changing iron content, impurities, and fluid chemistry during growth. The gray central area may be quartz, host rock, or an internal cavity fill.

Field identification & locations

Identify by its hardness, waxy appearance, curved/conchoidal fracture, and translucent layered color; common sources include Brazil, Uruguay, Mexico, India, Madagascar, and the United States. A photo alone cannot confirm exact variety.