mineral
Polished agate
Chalcedony (microcrystalline quartz, SiO₂), banded agate variety
AI-generated identification · may be incorrectHardness 6.5–7 Mohs; reddish-brown, cream, and gray bands with a waxy to vitreous luster. Microcrystalline quartz; no visible cleavage, conchoidal fracture. The stone is polished, with some natural-looking surface pits or inclusions.
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Physical properties
Hardness 6.5–7 Mohs; reddish-brown, cream, and gray bands with a waxy to vitreous luster. Microcrystalline quartz; no visible cleavage, conchoidal fracture. The stone is polished, with some natural-looking surface pits or inclusions.
Formation & geological history
Agate forms as silica-rich fluids fill cavities in volcanic rock, depositing chalcedony in successive bands. Formation ages vary widely, commonly from the Cenozoic or older.
Uses & applications
Used for jewelry, cabochons, ornaments, and inexpensive collector pieces; agate is also used for carvings and decorative objects.
Geological facts
Its layered bands record changes in the chemistry or conditions of silica deposition. The specimen’s brown and gray pattern is characteristic of common agate, though a photo cannot establish its exact variety.
Field identification & locations
Look for curved or concentric bands, translucency at thin edges, and hardness sufficient to scratch glass. Agates occur worldwide, especially in volcanic regions and river gravels.
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