Chert is a hard, fine-grained sedimentary rock composed almost entirely of microcrystalline quartz. This rock is classified as microcrystalline because its quartz crystals are so minute they cannot be seen without a high-power microscope. Because of its uniform, dense structure, chert breaks in a predictable manner, a property that was historically invaluable. Early human cultures across the globe recognized this durability and used chert extensively to fashion sharp-edged tools, such as scrapers, knives, and arrowheads.
Visual Identification: Color, Luster, and Texture
Chert exhibits a broad spectrum of colors, including various shades of gray, white, red, black, and green. For instance, chert with a reddish or brown hue, often called jasper, gets its coloration from iron oxides. Organic matter inclusions are responsible for the darker gray to black varieties, which are often referred to as flint.
The surface texture of chert is characteristically dense and smooth to the touch. A freshly broken surface usually displays a distinctive waxy or greasy luster, though some specimens may appear dull. This specific surface sheen is a significant visual clue that helps distinguish it from rocks with a glassy or vitreous appearance.
Testing Physical Properties
The most definitive method for identifying chert involves testing its physical properties, specifically its hardness and fracture pattern. Chert is a particularly hard rock, registering between 6.5 and 7 on the Mohs scale of mineral hardness. This high hardness means that chert will resist scratching by a steel nail or knife blade. A simple field test involves attempting to scratch a piece of glass, which chert can easily mark, confirming its high silica content.
Chert’s fracture is described as conchoidal, meaning shell-like. When struck, the rock breaks along smooth, curved surfaces that resemble the interior of a clam shell. This allows the rock to be flaked by toolmakers to produce exceptionally sharp edges, making the conchoidal fracture a primary diagnostic feature.
Geological Occurrence and Context
Chert is categorized as a sedimentary rock, typically forming through the accumulation and transformation of silica in marine environments. Much of the silica originates from the microscopic skeletons of marine organisms that settle on the seafloor. Over geologic time, this biological silica dissolves and recrystallizes into the dense microcrystalline quartz that forms chert.
The rock commonly occurs in two primary forms within larger rock formations. It is frequently found as irregularly shaped masses or nodules embedded within limestone or chalk beds. Alternatively, chert can be found as layered deposits, known as bedded chert, which are often interbedded with shale or other fine-grained sedimentary rocks.
Distinguishing Chert from Look-Alikes
Flint and Other Quartz Varieties
Flint, for example, is not a different material but is geologically defined as a specific variety of chert that is dark gray to black and found exclusively as nodules in chalk. Although the identification methods are identical, the term “chert” is generally used for lighter-colored or lower-quality specimens found in other types of limestone.
Distinguishing chert from macrocrystalline quartz involves observing the crystal structure, a difference that influences their appearance. While both are silicon dioxide, chert is composed of crystals too small to be seen, giving it a duller luster and smooth texture. Macrocrystalline quartz has visible crystals that result in a more brilliant, glassy luster. Chalcedony is another closely related microcrystalline variety of quartz, which is often included under the general term chert in petrology.
Volcanic Look-Alikes
Chert can also be mistaken for volcanic glass, such as obsidian, or dark igneous rocks like basalt. Obsidian is softer than chert, typically measuring between 5 and 6 on the Mohs scale, and it is also lighter in density. While both obsidian and chert exhibit conchoidal fracture, the higher hardness of chert and its sedimentary origin differentiate it from the volcanic glass. Basalt, on the other hand, is significantly softer and lacks the characteristic conchoidal fracture entirely.