gemstone

Polished gray agate or chalcedony

Microcrystalline quartz (SiO₂), likely gray chalcedony/agate

AI-generated identification · may be incorrect

Hardness 6.5–7 Mohs; gray to blue-gray, semi-translucent with cloudy brown inclusions and vitreous waxy luster. Trigonal quartz structure, no cleavage, conchoidal fracture; specific gravity about 2.60–2.65.

Identified September 15, 2026More gemstone →

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Physical properties

Formation & geological history

Forms when silica-rich groundwater fills cavities in volcanic rocks, precipitating layered or cloudy chalcedony over long periods, commonly during Cenozoic–Mesozoic volcanism. The specimen is tumbled and polished.

Uses & applications

Used for cabochons, beads, carvings, ornaments, and lapidary collections; durable enough for jewelry but less valuable than transparent quartz gems.

Geological facts

Agate is technically banded chalcedony, while massive unbanded material is usually called chalcedony. Brown or black zones may reflect iron oxides or other mineral inclusions.

Field identification & locations

Identify by its waxy luster, translucency at edges, conchoidal fracture, lack of cleavage, and hardness sufficient to scratch glass. Common sources include Brazil, Uruguay, Mexico, India, Madagascar, and the United States.