sedimentary or igneous

Quartz Geode (Nodule)

Silicon Dioxide (SiO2)

Hardness: 7 (Mohs scale), Color: White, clear, or off-white, Luster: Vitreous (glassy) to dull on the exterior, Crystal Structure: Hexagonal (trigonal), Cleavage: None (conchoidal fracture), Specific Gravity: 2.65

Identified March 30, 2026More sedimentary or igneous →

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Physical properties

Formation & geological history

Formed as a secondary mineral within hollow cavities in volcanic (igneous) or sedimentary rocks. Over geological time, mineral-rich groundwater seeps into these cavities, precipitating layers of microcrystalline or macrocrystalline quartz. Ages can range from thousands to hundreds of millions of years.

Uses & applications

Used primarily for ornamental and decorative purposes, gemstone collecting, metaphysical 'healing' crystals, and educational geological specimens.

Geological facts

Geodes are often referred to as 'thunder eggs' when filled with solid agate. Although they look like ordinary round rocks from the outside, they contain hollow centers lined with sparkling crystals. The world's largest geode is the 'Empress of Uruguay,' standing over 11 feet tall.

Field identification & locations

Identify in the field by looking for round or egg-shaped rocks that feel lighter than expected for their size (suggesting a hollow center). Common locations include volcanic ash beds and limestone deposits in regions like the American Midwest (Keokuk), Morocco, and Brazil.

Other sedimentary or igneous specimens

See all

Pebble (likely a type of sandstone or quartzite pebble)

Clastic Sediment (specifically, a rudite-sized clast). If the pebble is composed of a specific rock type, its scientific name would be that rock type (e.g., 'Quartzite pebble', 'Granite pebble', 'Chert pebble'). Given its appearance, it is likely a metamorphic quartzite pebble or a hard sandstone pebble.

Sedimentary rock

Quartz Vein in Host Rock

Silicon Dioxide (SiO2) within Sedimentary or Igneous Matrix

igneous

Fluorite with Quartz coating

Calcium Fluoride (CaF2) with Silicon Dioxide (SiO2)

mineral

Opal

Hydrated Silica (SiO2·nH2O)

mineral

Healer's Gold (Pyrite and Magnetite in Schist)

Magnetite (Fe3O4) and Pyrite (FeS2) in Mica Schist

metamorphic

Red Garnet (likely Almandine or Pyrope)

Garnet group: X3Y2(SiO4)3 (e.g., Almandine: Fe3Al2Si3O12)

mineral