sedimentary
Flint
Silicon Dioxide (SiO2) - Microcrystalline Quartz / Chert
Hardness: 7 on the Mohs scale. Color: Dark grey, black, or dark brown with a white chalky cortex. Luster: Vitreous to waxy. Crystal structure: Microcrystalline/cryptocrystalline (quartz). Cleavage: None, exhibits prominent conchoidal fracture. Specific gravity: 2.60 to 2.65.
- Hardness
- 7 on the Mohs scale
- Color
- Dark grey, black, or dark brown with a white chalky cortex
- Luster
- Vitreous to waxy
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Physical properties
Hardness: 7 on the Mohs scale. Color: Dark grey, black, or dark brown with a white chalky cortex. Luster: Vitreous to waxy. Crystal structure: Microcrystalline/cryptocrystalline (quartz). Cleavage: None, exhibits prominent conchoidal fracture. Specific gravity: 2.60 to 2.65.
Formation & geological history
Formed as nodules or concretions in sedimentary rocks such as chalk and marine limestones through the replacement of calcium carbonate by silica-rich solutions diagenetically.
Uses & applications
Historically used for making stone tools, arrowheads, and striking fire with steel. Today primarily used as a decorative stone, in construction aggregates, and for historical reenactments.
Geological facts
Flint is a variety of chert. When struck against steel, it produces sparks due to tiny bits of iron being sheared off and ignited by the friction, which historically made it essential for firearms (flintlocks) and fire-starting.
Field identification & locations
Easily identified in the field by its dark, glassy appearance, sharp conchoidal fracture edges, and the typically rough, white chalky outer layer (cortex). Found worldwide in chalk deposits.
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