sedimentary
Chert (and Agate/Chalcedony)
Microcrystalline Quartz (SiO2)
Hardness: 6.5-7 (Mohs scale), Color: Variegated brown, gray, and translucent white, Luster: Waxy to vitreous, Structure: Cryptocrystalline, Cleavage: None (conchoidal fracture), Specific Gravity: 2.6
- Hardness
- 6
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Physical properties
Hardness: 6.5-7 (Mohs scale), Color: Variegated brown, gray, and translucent white, Luster: Waxy to vitreous, Structure: Cryptocrystalline, Cleavage: None (conchoidal fracture), Specific Gravity: 2.6
Formation & geological history
Formed through the accumulation of silica-rich fluids in limestone or volcanic cavities. These specimens appear to be flint/chert nodules or agate-bearing geodes pieces found in sedimentary layers or alluvial deposits.
Uses & applications
Historically used for stone tools (arrowheads) due to conchoidal fracture. Today used for lapidary work, jewelry (especially the translucent agate portions), and as landscaping stone.
Geological facts
The sharp edges visible on some fragments demonstrate the rock's ability to break into blades, a property that made it the primary material for prehistoric survival tools.
Field identification & locations
Look for a waxy texture and shell-like (conchoidal) curves where the rock has broken. It is harder than steel and will scratch glass. Common in riverbeds and limestone outcrops.
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Cone-in-cone structure (Limestone)
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
Argillaceous sedimentary rock
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Sandstone
Arenite (primarily SiO2)
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