sedimentary
Chert (Flint / Agate Nodule)
Silicon Dioxide (SiO2)
Hardness: 6.5-7 on Mohs scale; Color: Dark brown, grey, black, with tan outer cortex; Luster: Waxy to vitreous; Crystal Structure: Microcrystalline/cryptocrystalline quartz; Cleavage: None (conchoidal fracture).
- Hardness
- 6
- Color
- Dark brown, grey, black, with tan outer cortex
- Luster
- Waxy to vitreous
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Physical properties
Hardness: 6.5-7 on Mohs scale; Color: Dark brown, grey, black, with tan outer cortex; Luster: Waxy to vitreous; Crystal Structure: Microcrystalline/cryptocrystalline quartz; Cleavage: None (conchoidal fracture).
Formation & geological history
Formed through the accumulation of silica-rich biogenic sediments or precipitated from groundwater within limestone or chalk beds, creating microcrystalline chalcedony nodules.
Uses & applications
Historically used for flintknapping tools and arrowheads; modernly used as decorative stones, in lapidary arts, and for rock tumbling.
Geological facts
Chert and flint were among the earliest materials used by early humans to fashion sharp tools, spear points, and fire-starting tools due to its sharp conchoidal fracturing behavior.
Field identification & locations
Identified by its hard, smooth, waxy surface, sharp shell-like (conchoidal) fractures, and rough chalky or weathered outer skin (cortex). Commonly found in sedimentary rock formations, gravel beds, and river banks worldwide.
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