sedimentary
Chert (River Cobble)
Chert (Cryptocrystalline Quartz, SiO2)
Mohs Hardness: 7, Color: Tan, brown, and yellowish-orange due to iron oxides, Luster: Waxy to dull, Crystal Structure: Microcrystalline, Cleavage: None (Conchoidal fracture), Specific Gravity: 2.6-2.7
Identified More sedimentary →
Identify your own rocks.
Get a report just like this from any photo, free.
Physical properties
Mohs Hardness: 7, Color: Tan, brown, and yellowish-orange due to iron oxides, Luster: Waxy to dull, Crystal Structure: Microcrystalline, Cleavage: None (Conchoidal fracture), Specific Gravity: 2.6-2.7
Formation & geological history
Formed by the precipitation of silica from water, often in marine environments or by replacing organic material. This specimen is a stream-rounded cobble, likely eroded from a limestone or dolomite source rock and transported by water.
Uses & applications
Historically used for stone tools (arrowheads, scrapers). Currently used as construction aggregate, road base, and occasionally for decorative landscaping.
Geological facts
Chert is essentially a dense, opaque variety of quartz. When it is red, it is called Jasper; when it is translucent and banded, it is called Agate. It is extremely chemically stable and resistant to weathering.
Field identification & locations
Identify in the field by its extreme hardness (cannot be scratched by a steel knife) and its smooth, shell-like (conchoidal) fracture when broken. Look for it in riverbeds, gravel pits, and sedimentary rock outcroppings.
More like this
Other sedimentary specimens
Sandstone or Siltstone
Arenite (if sandstone)
Sedimentary
Chert Breccia
Brecciated Chert (Microcrystalline Silica)
sedimentary
Sandstone
Arenite (primarily SiO2)
sedimentary
Sandstone Grain
Clastic Sedimentary Rock Grain (SiO2 dominated)
sedimentary
Shale or Slaty Mudstone
Argillaceous sedimentary rock
sedimentary
Sandstone
Arenite
sedimentary