sedimentary
Quartz Geode
Quartz (Silicon Dioxide - SiO2) within a Chalcedony and Limonite shell
Hardness: 7 on Mohs scale; Color: Clear to milky white internal crystals, grey or brown outer rind; Luster: Vitreous (glassy) inside, dull/earthy outside; Crystal Structure: Hexagonal (inside); Cleavage: None/Conchoidal; Specific Gravity: 2.65.
- Hardness
- 7 on Mohs scale
- Luster
- Vitreous (glassy) inside, dull/earthy outside
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Physical properties
Hardness: 7 on Mohs scale; Color: Clear to milky white internal crystals, grey or brown outer rind; Luster: Vitreous (glassy) inside, dull/earthy outside; Crystal Structure: Hexagonal (inside); Cleavage: None/Conchoidal; Specific Gravity: 2.65.
Formation & geological history
Formed in hollow cavities within sedimentary rocks (typically limestone) or volcanic ash. Mineral-rich groundwater seeps into these cavities over millions of years, precipitating layers of silica that crystallize into quartz. This specimen likely dates from the Carboniferous to Eocene periods depending on exact locality.
Uses & applications
Primarily used for decorative purposes, geological education, metaphysical collecting, and jewelry lapidary. Smaller geodes are often sold as 'break-your-own' kits for hobbyists.
Geological facts
Geodes are often referred to as 'Earth Eggs.' While they look like ordinary round stones from the outside, they contain hollow cavities lined with crystals. One of the largest geode sites in the world is the 'Geode Cave' in Ohio, but most commercial specimens like this one come from Morocco or the Mid-Western US.
Field identification & locations
Identify in the field by looking for rounded, lumpy, cauliflower-like rock nodules that are surprisingly light for their size (indicating a hollow center). Common locations include Keokuk, Iowa; Tabasco, Mexico; and Ouarzazate, Morocco. Collectors should look for a high ratio of clear crystals to host rock.
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