This is a piece of a chert nodule that I found in the Mojave Desert. It 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. Flint is a dark, hard chert like the rock that prehistoric stone tools are made from, and jasper is a bright, 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 agate.
Three big close-up pictures of chert are available as free wallpaper images.
For more photos see the Sedimentary Rocks Gallery.
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