mineral

Agate

Chalcedony (SiO2)

Hardness: 6.5-7 on Mohs scale; Color: Multi-colored bands (reds, oranges, grays, whites); Luster: Waxy to vitreous; Crystal Structure: Trigonal (microcrystalline); Cleavage: None; Specific Gravity: 2.58-2.64

Hardness: 6
Color: Multi-colored bands (reds, oranges, grays, whites)
Luster: Waxy to vitreous

Identified June 29, 2026More mineral →

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Physical properties

Formation & geological history

Formed primarily in volcanic and metamorphic rocks. It occurs when silica-rich groundwater fills cavities (vesicles) in igneous rocks, depositing layers of microcrystalline quartz over thousands of years. The distinct banding reflects changes in fluid chemistry.

Uses & applications

Commonly used for ornamental purposes, jewelry (cabochons, beads), decorative carvings, bookends, and precision instruments like mortars and pestles due to its hardness and resistance to chemicals.

Geological facts

Agate is named after the Achates River in Sicily where it was first discovered. The banding can be so thin that it takes 10,000 layers per inch. It is the state gemstone of several US states including Kentucky, Tennessee, and Montana.

Field identification & locations

Identify by looking for colorful, concentric banding within a rounded nodule that often has a rough, pitted exterior. Commonly found in riverbeds, gravel pits, and areas with ancient volcanic activity like Brazil, Uruguay, and the Lake Superior region.