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Chert ( //) is a fine-grained sedimentary rock composed of microcrystalline or cryptocrystalline silica, the mineral form of silicon dioxide (SiO2). Depending on its origin, it can contain either microfossils, small macrofossils, or both. It varies greatly in color (from white to black), but most often manifests as gray, brown, grayish brown and light green to rusty red (occasionally dark green too); its color is an expression of trace elements present in the rock, and both red and green are most often related to traces of iron (in its oxidized and reduced forms respectively).
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