sedimentary
Chert (Common Opal/Agate variety)
Microcrystalline Quartz (SiO2)
Hardness: 6.5-7 Mohs; Color: Brown, gray, and translucent tan; Luster: Waxy to vitreous; Structure: Cryptocrystalline; Cleavage: None (conchoidal fracture).
- Hardness
- 6
- Color
- Brown, gray, and translucent tan
- Luster
- Waxy to vitreous
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Physical properties
Hardness: 6.5-7 Mohs; Color: Brown, gray, and translucent tan; Luster: Waxy to vitreous; Structure: Cryptocrystalline; Cleavage: None (conchoidal fracture).
Formation & geological history
Formed via the accumulation of siliceous remains of organisms or chemical precipitation within sedimentary rock layers, often occurring as nodules in limestone or chalk.
Uses & applications
Historically used for stone tools due to sharp edges; currently used for landscaping, as a gemstone (when banded), and for decorative carvings.
Geological facts
Chert has been used by humans for millions of years to create arrowheads and scrapers because it breaks in predictable, razor-sharp flakes.
Field identification & locations
Identified by its waxy luster and smooth, curved (conchoidal) fracture surfaces. Commonly found in riverbeds and sedimentary outcrops globally.
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Cone-in-cone structure (Secondary Sedimentary Structure)
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Chert Breccia
Brecciated Chert (Microcrystalline Silica)
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Sandstone Grain
Clastic Sedimentary Rock Grain (SiO2 dominated)
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Shale or Slaty Mudstone
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Sandstone
Arenite (primarily SiO2)
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