Most chert forms as microcrystals of silicon dioxide grow within soft sediment in the Earth's crust. Some of the silicon dioxide in chert is thought to be fossilized matter. Meaning it once was part of living creatures. You can find more information here: http://geology.com/rocks/chert.shtml...

Chert is a type of sedimentary rock that contains silicon dioxide. Silicon dioxide, when dissolved in water, can crystallize out and grow into nodules called chert. You can find more information here: http://geology.com/rocks/chert.shtml...