mineral

Quartz geode

Silica (SiO₂), commonly chalcedony-lined quartz geode

AI-generated identification · may be incorrect

Hardness 7 for quartz and ~6.5–7 for chalcedony; typically gray, white, tan, or pale lavender with a glassy to waxy luster. Interior crystals are trigonal quartz; the specimen has a hollow cavity, botryoidal chalcedony rim, and no obvious cleavage.

Identified August 22, 2026More mineral →

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Physical properties

Formation & geological history

Forms when gas cavities in volcanic basalt or vesicular lava become lined by silica-rich groundwater, followed by inward quartz crystallization. Individual geodes may range from Cenozoic to much older, depending on the host volcanic rock.

Uses & applications

Used as décor, lapidary material, educational specimens, and occasionally jewelry; attractive crystal-lined pieces can be valuable collectibles.

Geological facts

The outer rind is usually microcrystalline chalcedony, while the cavity contains later-growing quartz crystals. Purple coloration, if genuine, indicates amethyst caused by trace iron and irradiation.

Field identification & locations

Identify by a hard, hollow, rounded nodule with a dull chalcedony rind and radiating crystals inside; common sources include Brazil, Uruguay, Mexico, Morocco, and the United States. Avoid soaking or harsh cleaning, which can damage coatings or fragile crystals.