sedimentary
Geode
Quartz / Chalcedony nodule
Hardness of 7 on the Mohs scale, grayish-brown outer weathering rind with a crystalline quartz interior cavity, vitreous to dull luster, cryptocrystalline to microcrystalline structure, specific gravity around 2.65.
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Physical properties
Hardness of 7 on the Mohs scale, grayish-brown outer weathering rind with a crystalline quartz interior cavity, vitreous to dull luster, cryptocrystalline to microcrystalline structure, specific gravity around 2.65.
Formation & geological history
Formed in cavities in sedimentary or igneous rocks where groundwater rich in silica and other minerals deposits crystals layer by layer over millions of years.
Uses & applications
Used primarily in lapidary arts, mineral collecting, and decorative display items.
Geological facts
Geodes start as hollow bubbles in rock, often formed by gas trapped in volcanic rock or animal burrows/roots in sedimentary beds, which are gradually filled with mineral-bearing water.
Field identification & locations
Often found in limestone formations or volcanic ash beds. Collectors typically look for rounded, unusually heavy nodules and crack them open to reveal the crystal lining.
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