mineral

Tumbled chalcedony/agate (likely botryoidal variety)

Microcrystalline quartz, SiO₂; possibly druzy chalcedony or common agate

AI-generated identification · may be incorrect

Hardness about 6.5–7 Mohs; gray to black with a waxy-to-glassy luster and a rounded, polished surface. Quartz is trigonal but occurs here as cryptocrystalline material; visible pale drusy texture may indicate tiny quartz crystals.

Identified September 18, 2026More mineral →

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Physical properties

Formation & geological history

Forms when silica-rich groundwater fills cavities and vesicles in volcanic rock, precipitating chalcedony and quartz in layers or botryoidal crusts. Most such material is Cenozoic to younger, though the specimen’s age cannot be determined from the image.

Uses & applications

Used for tumbling, cabochons, carvings, beads, and ornamental objects; common pieces have modest jewelry and collector value.

Geological facts

The dark color may reflect manganese, iron, carbonaceous impurities, or artificial dye/coating. A glassy conchoidal chip and resistance to scratching by a steel knife support quartz identification.

Field identification & locations

Common sources include Brazil, Uruguay, Mexico, India, Madagascar, and the western United States. Look for curved banding, translucent edges, waxy luster, and hardness greater than glass; visual identification alone is not definitive.