Amethyst Geode (Thunderegg)

Physical Properties

Hardness: 7 on Mohs scale. Color: Dark purple to blackish crystal center with gray/white chalcedony outer band. Luster: Vitreous (glassy) on crystals, waxy on outer rim. Hexagonal crystal system center, conchoidal fracture.

Formation & Geological History

Formed in volcanic ash beds or lava flows where gas bubbles and cavities filled with silica-rich groundwater. Over millions of years, crystals grew inward from the cavity walls. Found in Tertiary age volcanic deposits.

Uses & Applications

Primary use in jewelry, metaphysical collections, interior decoration, and as a popular lapidary material for hobbyists.

Geological Facts

Geodes are essentially 'pocket' environments. Small, dark amethyst geodes like this are frequently nicknamed 'black amethyst' when high in iron and radiation-driven color centers. They were often formed from bubbles in rhyolitic volcanic ash.

Field Identification & Locations

Identify in the field by looking for rounded, knobby, or dull-looking exterior rocks that feel lighter than expected for their size. Commonly found in Oregon (USA), Brazil, and Uruguay. Look for the characteristic bands and crystal centers when cracked or cut.

Identified on: 4/27/2026

Mode: Standard