sedimentary
Chert (Gray Chalcedony)
Microcrystalline Quartz (SiO2)
Hardness: 7 (Mohs), Color: Dark gray to bluish-gray, Luster: Waxy to vitreous, Crystal structure: Trigonal (microcrystalline/cryptocrystalline), Cleavage: None (conchoidal fracture), Specific gravity: 2.6-2.7.
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Physical properties
Hardness: 7 (Mohs), Color: Dark gray to bluish-gray, Luster: Waxy to vitreous, Crystal structure: Trigonal (microcrystalline/cryptocrystalline), Cleavage: None (conchoidal fracture), Specific gravity: 2.6-2.7.
Formation & geological history
Formed primarily through the chemical precipitation of silica-rich fluids. This specimen likely originated as a nodule within limestone or volcanic ash deposits where silica replaced organic material or carbonate minerals over millions of years.
Uses & applications
Used historically for stone tools due to its sharp conchoidal fracture. In modern times, it is used as a landscaping stone, in abrasive compounds, and as a semi-precious gemstone for decorative carvings and jewelry.
Geological facts
Chert and its varieties like flint were essential to early human technological development. This specific gray variety is often called 'Common Agate' or 'Gray Chalcedony' when it shows translucent properties.
Field identification & locations
Identify in the field by its smooth, shell-like (conchoidal) fracture pattern and its ability to scratch glass easily. Commonly found in river beds, beaches, and sedimentary rock outcrops globally.
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