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Chert

Pictures of Sedimentary Rock Types

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Chert is a sedimentary rock composed mostly of the mineral chalcedony—cryptocrystalline silica, or quartz in crystals of submicroscopic size. (more below)
Silica-based rock
Photo (c) 2005 Andrew Alden licensed to About.com
Chert can form in parts of the deep sea where the tiny shells of siliceous organisms are concentrated, or elsewhere where underground fluids replace sediments with silica. Chert nodules also occur in limestones. Learn more about chert.

This piece of chert that I found in the Mojave Desert shows chert's typical clean conchoidal fracture and waxy luster.

Chert may have a high clay content and look at first glance like shale, but its greater hardness gives it away. Also, the waxy luster of chalcedony combines with the earthy appearance of clay to give it the look of broken chocolate. Chert grades into siliceous shale or siliceous mudstone.

Chert is a more inclusive term than flint or jasper, two other cryptocrystalline silica rocks. See photos of all three in the chert picture gallery.

Three big close-up pictures of chert are available as free wallpaper images.

Other galleries:
Fossils
Geologic Features and Processes
Glaciers and Ice
Landforms
Minerals
Rocks
Geology and Society

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