Rock type

Chert (and flint)

Also called Flint, Hornstone

Chert (and flint)chert (sedimentary silica rock)All regions

2 photos

Description

Chert is a hard, fine-grained sedimentary rock made of microscopic quartz, and it is the most widely used chipped-stone material in places like Ohio. It forms mainly as nodules or beds in limestone and dolomite, where silica replaced the carbonate. Flint is the same stone: geologists save the word for a purer chert from places like the English chalk, but in America flint and chert are used as synonyms. Colour runs from white and gray through black, brown, green and red. It breaks with the shell-like conchoidal fracture that makes sharp edges, and heating often makes it waxier, darker or redder. Regional names such as Burlington, Fort Payne and Onondaga point to the formation it came from.

How to identify it

Colour: Gray, black and white are common; green, red, brown and other colours also occur (Maryland guide).
Texture: Microcrystalline or cryptocrystalline: the silica grains are too small to see. Finer-grained chert feels smooth and glassy, coarser chert sugary or gritty (Barry).
Luster: Dull or chalky through waxy to glassy; higher luster usually means finer grain (Barry).
Inclusions: Many cherts hold fossils such as crinoids, bryozoans, brachiopods and sponge spicules, plus dolomite crystals (see the named stone pages).
Cortex: Fresh nodules carry a softer chalky or limy primary cortex; cobbles long separated from their source develop a discoloured secondary skin (Bakken).
Heat treatment: Heating often makes chert waxier, darker or redder (Maryland guide). Modern knappers heat-treat flint for a smoother, waxy texture and brighter colours as iron oxidises, though how it works is not settled (Getz).

Geology

Most nodular chert forms when silica replaces limestone; bedded chert forms when carbonate beds are completely replaced, or when siliceous ooze (partly from sponge spicules) is altered. Where the silica comes from is not settled (Ohio Geological Survey). Chert occurs in thin beds or irregular nodules in sedimentary carbonate rocks such as limestone, and cobbles washed out of those formations lie along streams. In Ohio alone flint ranges through rocks from the Silurian Brassfield Limestone to the Pennsylvanian Cambridge limestone. True flint, in the narrow geological sense, is found in only a few places such as the English chalk.

Where it comes from

All regions

Quarries and outcrops

Found throughout the United States wherever limestone and dolomite occur; see the named pages (for example Burlington, Onondaga, Fort Payne, Flint Ridge and Edwards cherts) for sources

Used by

Flint was the most used rock type throughout Ohio prehistory, and flintknapping can be traced back further than any other technology (Getz)

Look-alikes

Stones that are easy to confuse with Chert (and flint).

Jasper (iron-rich, opaque chert)
Chalcedony and agate (translucent, fibrous quartz)
Quartzite and silicified sandstone (break through sand grains and look sugary)
Silicified wood and silcrete

Sources

Getz, Garry L. 2016 (rev.). Flint: Ohio's Official Gemstone. Educational Leaflet No. 6. Ohio Department of Natural Resources, Division of Geological Survey.dam.assets.ohio.gov (opens in a new tab)
Maryland Archaeological Conservation Laboratory. Chert. Diagnostic Artifacts in Maryland: Common Stone Types (Jefferson Patterson Park and Museum).apps.jefpat.maryland.gov (opens in a new tab)
Delaware Department of Transportation. Archaeology glossary.deldot.gov (opens in a new tab)
Bakken, Kent E. (written about 1995; PDF issued later). Chipped Stone Raw Materials. Chapter 6 in Stone Tools of Minnesota. Minnesota Office of the State Archaeologist, St. Paul.mn.gov (opens in a new tab)
Barry, Steven Edward. 2004. Lithic Raw Materials and Chipped Stone Tools: A Comparison of Two Sites in the Moundville Chiefdom. M.A. thesis, University of Alabama, Tuscaloosa.rla.unc.edu (opens in a new tab)

Named stones of this kind

Points made from this stone

All materials