mineral

Chalcedony (gray agate/jasper-like nodule)

Microcrystalline quartz, SiO₂; likely iron-stained chalcedony

AI-generated identification · may be incorrect

Hardness 6.5–7 Mohs; waxy to dull luster, gray-blue translucent body with rusty red-brown iron-oxide staining. Trigonal quartz structure but crystals are microscopic; conchoidal fracture and no cleavage, with specific gravity about 2.6.

Identified September 10, 2026More mineral →

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Physical properties

Formation & geological history

Forms when silica-rich groundwater fills cavities, fractures, or vesicles in volcanic rocks, commonly during the Cenozoic or older hydrothermal episodes. The reddish patches represent later oxidation of iron-bearing minerals.

Uses & applications

Used as lapidary rough, cabochons, beads, carvings, and decorative stone; less valuable when opaque, fractured, or heavily weathered.

Geological facts

Chalcedony is a cryptocrystalline quartz aggregate; “agate” generally implies visible banding, while “jasper” is more opaque and impurity-rich. This specimen appears closer to massive gray chalcedony than strongly banded agate.

Field identification & locations

Identify by its waxy appearance, very high hardness, conchoidal fracture, and inability to scratch it with a steel knife easily; it should scratch ordinary glass. Common in volcanic terrains worldwide, especially Brazil, Uruguay, Mexico, India, and the western United States.