Chert is characterized by being harder than glass, brittle, very fine-grained, and by breaking with a smooth, rounded or hollowed clam shell-like (conchoidal) fracture and sharp edges. It was used by Indians to make arrow heads. It accumulates in abundance both in stream beds as gravel which has been more or less rounded by wear, and on the hillsides within the soil and sub-soil. Yellow and red iron oxides may stain and penetrate weathered chert gravel so that it becomes reddish, rusty, tan, yellow or brown.
Chert remains abundant because of its extreme resistance to weathering. It is so hard that stream action wears it only very slowly. Its chemical composition is silica, SiO₂, a substance which is but little affected chemically by ground water. Where chert has contained fine grains of calcite scattered through it, the calcite may be removed in solution, leaving pores, and a zone of porous, light weight, tripolitic chert, harsh to the feel and enveloping an unaltered interior (See [WEATHERED CHERT]). Not uncommonly, fossil remains of calcite which were embedded in chert have been dissolved, leaving their hollow impressions.
[Chert] in Missouri originally occurs chiefly in limestone formations, where it is found as nodules, lenses, stringers, and irregular forms in and between the limestone beds. Chert and flint may be deposited directly from silica in solution, or they may replace (substitute for) wood, fossils, or older rock where silica-bearing solutions contact and react with the replaced substance. For example, petrified wood usually is wood which has been replaced molecule by molecule with silica. This statement applies equally to the brightly colored petrified wood in the Petrified Forest in Arizona and to that with comparatively drab coloring in Missouri. Many other siliceous fossils, notably animal remains, are replacements of calcite (limestone) by silica.
In anticipation that the reader may have difficulty understanding how silica may go into solution if chert (silica) is hardly attacked by the weathering process, it should be explained that silica is freed in solution predominately during the weathering of complex silica-combinations, silicates, rather than from uncombined silica. For instance, feldspar and pyroxene from granite or gabbro weather in ground water to a soil-forming clay mineral and release some silica in solution in the ground water. After this silica is redeposited in an uncombined form, like chert, it becomes highly insoluble.
An observation in regard to flint is that the metallic “flints” which are used to ignite gas burners or cigarette lighters are not black chert, SiO₂. Instead, they are special alloys containing rather uncommon elements which possess the useful characteristic of emitting a brilliant hot spark when harshly scratched.
Chert is the chief source of natural gravel in Missouri because it accumulates in stream beds and on hillsides on account of its resistance to weathering. The piles of “chats” in the Joplin region, containing thousands of tons of crushed chert, have been used in part in road surfacing material.
Weathered Chert
Weathered chert, or leached chert, is a white to gray, or yellowish, porous, light-weight, harsh to feel, chalky-appearing rock which occurs over much of the southern half of Missouri. It does not effervesce in acid. Usually it occurs as a zone from a fraction of, to more than an inch in thickness, about a denser core of hard, compact chert (flint), or makes up an entire small rock fragment or gravel.
Chert hand specimen showing quartz-lined fossil cavity in center, compact fresh chert in interior, and chalky-appearing weathered outside margins. From near Columbia.