gemstone

Polished amethyst geode

Quartz, SiO₂ (purple variety: amethyst), with banded chalcedony/agate

AI-generated identification · may be incorrect

Mohs hardness 7; vitreous luster on crystals and waxy luster on chalcedony; purple quartz crystals lining a cavity with gray-brown concentric banding. Quartz is trigonal and lacks cleavage, showing conchoidal fracture; specific gravity about 2.65.

Identified August 26, 2026More gemstone →

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Physical properties

Formation & geological history

Silica-rich fluids filled cavities in volcanic basalt, where quartz and amethyst crystallized in stages, often producing agate bands. Most commercial examples formed in Mesozoic–Cenozoic volcanic provinces, especially Brazil and Uruguay.

Uses & applications

Used as a decorative polished specimen, jewelry, and lapidary material; larger geodes are display pieces. Common polished specimens typically sell for about $20–$150, depending on size, color, crystal quality, and provenance.

Geological facts

Amethyst’s purple color commonly results from iron impurities modified by natural irradiation. Heating can turn it yellow-orange, producing citrine.

Field identification & locations

Identify it by purple transparent-to-translucent quartz crystals, hexagonal crystal faces, and associated gray chalcedony bands; it should scratch glass but not be scratched by a steel blade. Common sources include Brazil, Uruguay, Zambia, and Mexico.