THE LEARNING CENTER
Flint Verses Chert: What's the difference?
Contributed by member Michael Rogers
One of the most common questions asked by artifact collectors is: "What is the difference between flint and chert"? This article attempts to answer that question from a geological and chemical perspective.
Minerals The Silica Group
Minerals are the natural crystalline materials that form the Earth and make up most of its rocks. The most important group of minerals involved in the formation of flints/cherts is the Silica Group. This large group includes all minerals with the primary chemical formula 5i02 (silicon dioxide) and is most abundantly represented in nature by pure quartz and its many cryptocrystalline forms.
Cryptocrystalline quartz is simply quartz whose crystals are so small that they can only be seen with the aid of a high-power microscope. It is formed geologically from silica that has dissolved from silicate materials. Over geological time, this amorphous silica gel dehydrates to form microscopic crystals and eventually becomes what we know physically as rock. Cryptocrystalline quartz occurs in many varieties. These varieties have been named based on their color, opacity, banding and other observable physical features. Technically speaking, the two varieties that account for the vast majority of "flint" artifact materials are chalcedony and chert.
Other varieties encountered in the artifact world are agate, jasper and petrified wood. Interestingly, petrified wood is usually wood that has been replaced by agate. This same process also occurs with coral, hence the term "Agatized coral".
Chalcedony Chert and Flint
Chalcedony is a variety of cryptocrystalline quartz with extremely small crystals and a specific gravity (weight under water, a measure of a rock/mineral's purity) nearly identical to that of pure quartz. Due to its very high quartz content and super fine particle matrix, chalcedony has a very waxy luster.
Chert is composed of larger crystal particles and has a specific gravity similar that of pure quartz. Due to impurities and larger particle sizes, chert is somewhat less "quartz-like" than chalcedony. Chert is duller and more opaque than chalcedony and its luster ranges from non-existant to very waxy, depending on the individual rock formation.
So what is flint? By mineralogical definition, flint is simply black chert. It appears that the term "flint" was originally applied to the high quality black cherts found in England. Over the years names have evolved for local chert formations/deposits that may include the word "flint" and technically speaking these would be incorrect more often than not. The reality of the flint verses chert debate is that in most cases it is something like "splitting hairs", there really is very little difference, chemically speaking. Artifact collectors tend to call materials that have a more waxy luster "flints" and those which have less luster to no luster "cherts". The difference between them lyes in their purity relative to pure quartz and their matrix particle size. The smaller the particle size and the purer the material, the more likely we collectors would be to call the material flint. To a purist, we would be wrong. A generalist would say "close enough".
Note: Some examples of Flint Ridge Flint are known to be 98.93 % pure silicon dioxide.
Sorrell Charles A., Minerals of the World: A Field Guide and Introduction to the Geology and Chemistry of Minerals., Golden Press, New York, 1973 pp 206-209.