sedimentary
Chert (Fossiliferous)
Chert (Cryptocrystalline Silica/SiO2)
Hardness: 7 (Mohs scale), Color: Gray with tan inclusions, Luster: Waxy to dull, Crystal structure: Cryptocrystalline, Cleavage: None (conchoidal fracture), Specific Gravity: 2.65
Identified More sedimentary →
Identify your own rocks.
Get a report just like this from any photo, free.
Physical properties
Hardness: 7 (Mohs scale), Color: Gray with tan inclusions, Luster: Waxy to dull, Crystal structure: Cryptocrystalline, Cleavage: None (conchoidal fracture), Specific Gravity: 2.65
Formation & geological history
Formed through the accumulation of silica-rich marine organisms or chemical precipitation within limestone/chalk beds, often during the Paleozoic or Mesozoic eras. The weathering of softer host rock leaves behind these durable nodules.
Uses & applications
Historically used for stone tools/arrowheads. Modern uses include decorative landscaping, road base, and occasionally as semi-precious gemstones when displaying unique patterns.
Geological facts
This specimen appears to be a water-worn nodule containing trace fossils or vugs where fossils were once present. Chert is so hard it can strike sparks from steel, a property used by early humans for fire-starting.
Field identification & locations
Identify in the field by its hardness (it will scratch glass), waxy feel, and smooth, rounded edges if water-worn. Found globally in areas with sedimentary basin legacies, especially riverbeds and shorelines.
More like this
Other sedimentary specimens
Sandstone or Siltstone
Arenite (if sandstone)
Sedimentary
Sandstone
Arenite (primarily SiO2)
sedimentary
Chert Breccia
Brecciated Chert (Microcrystalline Silica)
sedimentary
Sandstone Grain
Clastic Sedimentary Rock Grain (SiO2 dominated)
sedimentary
Shale or Slaty Mudstone
Argillaceous sedimentary rock
sedimentary
Sandstone
Arenite
sedimentary