Chert

Written by Boss Tambang Tuesday, 01 December 2009 23:08

Gallery - Rock Gallery

 

 

Striped chert pebbles like these are common along parts of the California coast. Chert is a rock type composed mostly of the mineral chalcedony—cryptocrystalline silica, that is, quartz in crystals of submicroscopic size. It can form in parts of the deep sea where the tiny shells of siliceous organisms are concentrated, or where underground fluids replace sediments with silica. Chert nodules also occur in altered limestones.

Chert is a more inclusive term than flint or jasper, both of which are also cryptocrystalline silica rocks. Flint is used for dark, hard cherts like those that prehistoric stone tools are made from, and jasper is a reddish chert associated with iron-rich deposits. All of these have a similar waxy luster, and a hardness and density near that of quartz. Silica rocks of greater purity, translucency, and refined appearance tend to be classified as agates.

These pebbles are layered with impurities, probably clay minerals or volcanic ash or silty sediment—it’s hard to tell which without a microscope. The one on the left was fractured and offset during its time underground. The chert boulder below shows a more plastic deformation, which occurred at greater depth as part of the Sierra Nevada’s history.

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