Chert (IPA: /ˈtʃəː(r)t/) is a fine-grained silica-rich cryptocrystalline sedimentary rock that may contain small fossils. It varies greatly in color from white to black, but most often manifests as gray, brown, grayish brown and light green to rusty red; 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). Jasper is basically chert which owes its red color to iron(III) inclusions.
Chert outcrops as oval to irregular nodules in greensand, limestone, chalk, and dolostone formations as a replacement mineral, where it is formed as a result of diagenesis. It also occurs in thin beds, when it is a primary deposit. Thick beds of chert occur in deep geosynclinal deposits. These thickly bedded cherts include the novaculite of the Ouachita Mountains of Arkansas, Oklahoma, and similar occurrences in Texas in the United States. The banded iron formations of Precambrian age are composed of alternating layers of chert and iron oxides.
Chert is generally considered to be less attractive and more common than flint, although the two materials are closely related. In geological terms flint and chert are the same, with the term flint referring to chert found in chalk.

Chert and flint: archaeological and historical uses

Chalcedony
Eolith
Flint
Obsidian
List of minerals