Chert, one of the main forms of silicon dioxide, is cryptocrystalline (microscopic) quartz. Most of the chert in Illinois is white, gray, or black, but impurities stain many deposits yellow, brown, or even pink. Chert is so hard that it can scratch glass and ordinary steel. It is fairly lightweight, opaque, dull, and brittle. Specific gravity is about 2.6 to 2.64.

Flint, a variety of chert, is generally dark colored, is more dense, may have a glassy luster, and may be translucent in thin flakes. Both chert and flint have a smooth, curved (conchoidal) fracture, but flint tends to break with thinner, sharper edges. Native Americans used flint and chert to make arrow points and spearheads. Archeologists reserve the term flint for the dark varieties of chert from the Old World and use the term chert for the New World varieties.

Chert occurs as rounded masses (nodules and concretions) or as irregular layers in limestones and dolomites throughout Illinois. Because chert is hard and more resistant to weathering than limestone or dolomite, it often remains after the rest of the rock has weathered away. Chert also is abundant in many glacial deposits because it is hard and resistant to erosion. Streams that flow through cherty bedrock or glacial deposits carry pebbles along and concentrate them as gravel in stream channels. Cherty stream gravels are especially abundant in western and southern Illinois.

Ancient brown chert gravels in the southern part of the state are used for road gravel. Other deposits in extreme southern Illinois, consisting of angular fragments of chert and a small amount of clay (known locally as novaculite gravel), also are used for road surfacing, but chert reacts adversely with cement and cannot be present in aggregate used for roads or foundations.

White and gray chert occurs as thick lenses within massive bedrock deposits several hundred feet thick in Union and Alexander Counties.