sedimentary
Chalcedony / Agate Nodule
Silicon Dioxide (SiO2)
Mohs hardness of 6.5 to 7; color ranges from grey, white, to brownish tones; vitreous to waxy luster; cryptocrystalline structure; conchoidal fracture with no true cleavage; specific gravity of 2.58 to 2.64.
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Physical properties
Mohs hardness of 6.5 to 7; color ranges from grey, white, to brownish tones; vitreous to waxy luster; cryptocrystalline structure; conchoidal fracture with no true cleavage; specific gravity of 2.58 to 2.64.
Formation & geological history
Formed through the deposition of silica-rich groundwater solutions in cavities or fractures of rocks, typically volcanic or sedimentary, over millions of years.
Uses & applications
Used in ornamental carvings, lapidary arts, jewelry, and prized by mineral collectors.
Geological facts
Chalcedony is a cryptocrystalline form of silica, meaning its crystals are too small to be seen without high magnification. Nodules often form hollow cavities lined with tiny quartz or chalcedony crystals, creating mini-geodes.
Field identification & locations
Can be identified in the field by its waxy luster, conchoidal fracture, and resistance to scratching steel. Frequently found in regions with historical volcanic activity or limestone formations containing silica nodules.
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