mineral
Polished purple banded agate geode
Cryptocrystalline quartz (chalcedony), SiO₂, with purple coloration likely from amethyst/quartz and possible dye enhancement
AI-generated identification · may be incorrectHardness 6.5–7 Mohs; concentric violet, white, and gray chalcedony bands; vitreous to waxy luster; translucent edges; central cavity lined with tiny quartz crystals. No obvious cleavage; specific gravity about 2.6.
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Physical properties
Hardness 6.5–7 Mohs; concentric violet, white, and gray chalcedony bands; vitreous to waxy luster; translucent edges; central cavity lined with tiny quartz crystals. No obvious cleavage; specific gravity about 2.6.
Formation & geological history
Forms when silica-rich groundwater fills gas bubbles or cavities in volcanic rock, depositing chalcedony in successive bands and later quartz crystals. Usually Cenozoic or older, depending on its volcanic host; this polished slab may be dyed, a common treatment for vivid purple agate.
Uses & applications
Used as a decorative slab, paperweight, lapidary material, cabochon, and jewelry ornament. Common specimens typically sell for about $10–$60; larger, strongly colored polished pieces may reach $75–$200.
Geological facts
The concentric bands record changing chemistry and flow conditions during cavity filling. Purple agate is frequently color-enhanced, so intense uniform violet should not automatically be interpreted as natural amethyst.
Field identification & locations
Identify by curved, parallel chalcedony bands, a hard glassy surface, and a drusy quartz-lined center. Agates are widespread in volcanic terrains; comparable material occurs in California and the western United States, though the exact locality cannot be determined visually.
