CHERT, one of the main forms of silicon dioxide, is cryptocrystalline
(submicroscopic) quartz. Most of the chert in Illinois is white or gray,
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 light
weight, dense, opaque, and brittle; the luster is dull.
Flint, a variety of chert, is generally dark colored, more dense, may
have a glassy luster and be transparent in thin flakes. Both chert and
flint have a smooth, curved (conchoidal) fracture, but flint tends to
break with thinner, sharper edges. Indians used flint and chert to make
arrow points and spearheads.
Chert occurs as rounded masses (nodules and concretions) or as irregular
layers in limestones and dolomites in northern, western, southwestern,
and southern 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
resists solution. 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.
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.
White and gray chert occur as massive bedrock deposits several hundred
feet thick in Union and Alexander Counties.