sedimentary
Chert (Gravel Bit)
Microcrystalline silica (SiO2)
Hardness: 7 (Mohs scale); Color: Brown/Opaque; Luster: Waxy to dull; Structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.65
- Hardness
- 7 (Mohs scale)
- Color
- Brown/Opaque
- Luster
- Waxy to dull
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Physical properties
Hardness: 7 (Mohs scale); Color: Brown/Opaque; Luster: Waxy to dull; Structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.65
Formation & geological history
Formed through the precipitation of silica from groundwater or the accumulation of siliceous skeletal remains of microorganisms (like radiolaria) in marine environments. This specimen appears to be a weathered fragment of common river or construction gravel.
Uses & applications
Common varieties are used in road construction, concrete aggregate, and landscaping. Historically, varieties like flint were used to make sharp tools and arrowheads.
Geological facts
Chert is so hard it can scratch steel and glass. It is chemically identical to quartz but has a much finer, microscopic grain structure that makes it appear opaque rather than translucent.
Field identification & locations
Identify in the field by its hardness and its tendency to break with smooth, curved 'shell-like' surfaces (conchoidal fracture). It is found globally in sedimentary rock layers or as loose gravel in riverbeds.
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