mineral

Polished agate slab

Microcrystalline quartz (chalcedony), SiO₂, with iron-oxide staining

AI-generated identification · may be incorrect

Hardness 6.5–7 Mohs; translucent gray, cream, and salmon-orange bands with a waxy to glassy luster. It has cryptocrystalline quartz structure, no obvious cleavage, and conchoidal fracture; specific gravity is about 2.6.

Identified September 13, 2026More mineral →

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Physical properties

Formation & geological history

Agate forms when silica-rich groundwater fills cavities in volcanic rocks, depositing chalcedony and quartz in successive bands; iron oxides create the orange-red coloration. Individual deposits range from Paleozoic to Cenozoic in age.

Uses & applications

Used for cabochons, carvings, bookends, ornamental slabs, and lapidary collectibles; it is durable but generally modestly priced unless exceptionally patterned or vividly colored.

Geological facts

The visible curved bands reflect changing chemistry and flow conditions during cavity filling, sometimes producing fortification-like patterns.

Field identification & locations

Identify it by concentric or curved translucent banding, waxy appearance, and glass-like fracture; common sources include Brazil, Uruguay, Mexico, India, and the western United States. Estimated value: $10–$40 for a common polished slab, or $50–$150+ for larger, highly attractive material.