mineral

Amethyst geode slice

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

AI-generated identification · may be incorrect

Hardness 7 Mohs; purple druzy quartz crystals enclosed by gray-white chalcedony bands and dark rind. Vitreous to sugary luster, trigonal quartz crystals, no cleavage, specific gravity about 2.65.

Identified August 26, 2026More mineral →

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Physical properties

Hardness 7 Mohs; purple druzy quartz crystals enclosed by gray-white chalcedony bands and dark rind. Vitreous to sugary luster, trigonal quartz crystals, no cleavage, specific gravity about 2.65.

Formation & geological history

Forms when silica-rich groundwater fills vesicles in volcanic basalt; successive chalcedony bands and later amethyst crystals line the cavity. Most commercial examples formed in Cretaceous–Cenozoic volcanic rocks, commonly in Brazil and Uruguay.

Uses & applications

Used as decorative slices, display specimens, jewelry, and lapidary material; fine saturated crystals have greater value than pale or heavily included pieces.

Geological facts

Amethyst’s color is caused mainly by iron impurities and irradiation. Heating can turn it yellow-orange, producing citrine-like material.

Field identification & locations

Identify by concentric agate banding, a volcanic-looking outer rind, and sparkling purple quartz lining the cavity; glass lacks natural banding and quartz crystal texture. Major sources include Brazil, Uruguay, Zambia, and Bolivia.