Chert is the proper name for the vast majority of raw materials from which the hafted bifaces ("arrowheads") and other stone tools of east-central Illinois were fabricated. In Europe similar appearing material would be referred to as flint. Both are rich in silicon dioxide (SiO2). A modern industrial expression of relatively pure silicon dioxide is window glass. You might want to think of chert as window glass with impurities and a slightly different molecular arrangement.
Many types of chert were exploited by the prehistoric people of the area. This condition likely reflects both the size of their territories and the effects of trade and exchange. The assignment of a particular object to a particular type category is fraught with many difficulties. There is much overlap between type criteria and there is much variation in chert coming from a particular source. What we will try to do in the type key is make use of those qualities of a particular chert that are conspicuous to the unaided eye, distinctive to the chert, and typical of a source. A majority of artifacts should be assignable to a chert type.
The type key is based on five properties of chert: texture, luster, color, fossils, and structural characteristics.
Texture: While the crystals comprising the matrix of chert are quite small, they are nonetheless variable. The categories we employ are coarse, medium, and fine. Coarse would mean that the crystals are visible to the unaided eye and the surface would be rough to the touch; medium requires 8X magnification to view the crystals and the fingernail grates on the surface (Rick 1978:15); and fine requires the use of a microscope to view individual crystals while the surface would feel not quite as smooth as glass but almost.
Luster: Luster has to do with how light is reflected from the surface of the object. The key employs three categories: dull, waxy, and glassy. Dull means that little or no light will be reflected, a familiar example might be chalk. Waxy means that some light will be reflected as it is from the lip or rim of a used candle. Glassy luster is as the label implies - glass like.
Color: Communicating color by verbal description is always difficult. To address this difficulty, archaeologists use the Munsell color charts to create a standard of reference. Most users of our site will not have access to the Munsell charts, so we will do our best (although the library of your regional university will likely have them). Of course, one of the great advantages of our electronic medium is that the user can clearly and easily see the color that we are trying to describe.
Fossils: Several of the chert types require recognition of certain index fossils. An illustration of the common forms can be found at fossils. The image is at a magnification of about 10X and is about 500K in size.
Structural Characteristics: Some cherts will display structural characteristics that are useful in their identification. Some, like voids (also refered to as cavities or vugs) or iron oxide (rust) lined voids are rather commonly understood, others like druse and oolites may require some illustration.
While we don't employ it as a sorting criterion, heat treatment or thermal alteration was a technology applied to several types of chert. The macroscopic effect was to alter color and/or luster. We will provide descriptions of the changes as they are known and appropriate.
A final point of confusion for those of us working in east-central Illinois is that much of the chert being exploited was recovered from glacial deposits. Glacial deposits contain EVERYTHING. Consequently, Glacial Till Chert represents our "Other" or catch-all category. One should probably begin an analysis by reviewing the properties of Glacial Till Chert.
To start the process of identification, click on the "CHERT TYPE KEY" above.
To learn about specific chert types, click on the "CHERT INDEX" above.