sedimentary
Chert (Gravel / Pebble)
Microcrystalline or Cryptocrystalline Quartz (SiO2)
Hardness: 6.5-7 on Mohs scale. Color: Tan, brown, grey, reddish. Luster: Waxy to dull. Crystal structure: Trigonal (microscopic). Cleavage: None (conchoidal fracture). Specific gravity: 2.65.
- Hardness
- 6
- Color
- Tan, brown, grey, reddish
- Luster
- Waxy to dull
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Physical properties
Hardness: 6.5-7 on Mohs scale. Color: Tan, brown, grey, reddish. Luster: Waxy to dull. Crystal structure: Trigonal (microscopic). Cleavage: None (conchoidal fracture). Specific gravity: 2.65.
Formation & geological history
Formed through the precipitation of silica from seawater or the replacement of limestone. These specific rounded pebbles are common in glacial till or river bed deposits from the Quaternary period.
Uses & applications
Used in road construction, concrete aggregate, landscaping gravel, and historically for stone tools due to its sharp edges when broken.
Geological facts
Chert is so hard it can strike sparks against steel. A variety of chert called flint was essential for prehistoric tool making and survival.
Field identification & locations
Identify in the field by its hardness (it will scratch glass) and its curved, shell-like (conchoidal) fracture surfaces. Commonly found in driveway gravel or creek beds throughout North America.
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