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.