sedimentary
Chert (often called Flint)
Microcrystalline Quartz (SiO2)
Hardness: 7 on Mohs scale; Color: typically tan, grey, or black with waxy luster; Crystal Structure: Cryptocrystalline (microscopic); Cleavage: None (conchoidal fracture); Specific Gravity: 2.65.
- Hardness
- 7 on Mohs scale
- Color
- typically tan, grey, or black with waxy luster
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Physical properties
Hardness: 7 on Mohs scale; Color: typically tan, grey, or black with waxy luster; Crystal Structure: Cryptocrystalline (microscopic); Cleavage: None (conchoidal fracture); Specific Gravity: 2.65.
Formation & geological history
Formed as nodules or layers in limestone through the accumulation of silica-rich organic remains (like diatoms or radiolarians) or through the chemical replacement of host rocks. Common in deposits ranging from the Precambrian to Cenozoic.
Uses & applications
Used historically for stone tools and fire-starting; currently used in construction aggregate, road surfacing, and as a semi-precious stone in jewelry (when decorative).
Geological facts
Chert has a conchoidal fracture, allowing it to break into sharp edges, which made it the primary material for arrowheads and spear points for thousands of years. It can come in various colors such as red (jasper) or translucent (chalcedony).
Field identification & locations
Identify by its 'waxy' appearance, extreme hardness (scratches glass), and the way it breaks into smooth, shell-like curves. Widely found in limestone beds and river gravels worldwide.
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