mineral

Quartz-lined geode

Silica geode; predominantly microcrystalline quartz (SiO₂), possibly chalcedony

AI-generated identification · may be incorrect

Hardness ~7 Mohs; gray to brown, with a dull chalcedony rind and sparkling white-to-gray quartz crystals inside; vitreous luster, trigonal quartz structure, no prominent cleavage, specific gravity ~2.65.

Identified August 26, 2026More mineral →

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Physical properties

Formation & geological history

A cavity in volcanic or sedimentary rock was progressively filled by silica-bearing groundwater, first forming chalcedony and later quartz crystals. The exact geological age cannot be determined from the photograph; geodes commonly form in volcanic flows and sedimentary carbonate deposits.

Uses & applications

Used as a decorative specimen, display piece, and lapidary material; attractive crystal-lined examples are collected and occasionally cut into bookends or polished sections.

Geological facts

The dark outer rind and hollow crystalline interior are characteristic of a geode rather than a solid agate nodule. The crystals may include drusy quartz, with possible minor calcite or iron-oxide staining.

Field identification & locations

Identify by its rounded weathered exterior, hollow cavity, chalcedony banding, and inward-pointing crystals; quartz will scratch glass but not be scratched by a steel knife. Common sources include Brazil, Uruguay, Mexico, Morocco, and the western United States.