Chert
“Silica from the seafloor”
Sedimentary
Chert is a hard, microcrystalline sedimentary rock composed of silica, often forming from the accumulated siliceous skeletons of radiolarians, diatoms, and sponges on the deep ocean floor. It occurs as nodules in limestone or as bedded deposits.
Composition
microcrystalline quartz (chalcedony), opal, traces of organic matter
Where Found
Bighorn Mountains (Wyoming, USA), Southern England, Japan (bedded cherts), Australia
Mohs Hardness
6.5-7
Fun Fact
Geologists use chert layers called 'Banded Iron Formations' to study Earth's ancient atmosphere — they record when oxygen first appeared in our air 2.4 billion years ago.
Test Your Knowledge
Why are banded chert formations considered key records of early Earth's atmosphere?
Related Specimens
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