Chert

Chert is a hard, dense sedimentary rock composed almost entirely of microcrystalline (and sometimes amorphous) silica, essentially a fine-grained variety of quartz. It occurs in a wide range of colors depending on impurities, from white and grey to black, brown, green, and red.

Because it is very hard (Mohs 7) and breaks with a smooth, sharp conchoidal fracture, chert was one of the most important raw materials in prehistory for making tools, blades, and arrowheads. Flint is essentially a dark variety of chert, and jasper is a red or colored, iron-rich variety.

Chert occurs both as nodules within other sedimentary rocks (notably limestone) and as continuous bedded layers formed from silica-rich sediments.

Chert forms in two main ways. Bedded chert often forms from the accumulation of silica-rich skeletons of microscopic organisms such as radiolarians and diatoms on the deep seafloor, which later recrystallize into solid silica rock.

Nodular chert forms by chemical replacement, when silica-bearing groundwater precipitates within host sediments, commonly limestone and chalk, replacing the carbonate and growing irregular nodules and lenses.

Chert is found worldwide. Famous examples include the bedded radiolarian cherts of California's Franciscan Complex, the flint nodules in the chalk of England and France, the Arkansas novaculite (a very pure chert), and abundant chert in limestone formations across North America and Europe.

Look for a very hard, dense, fine-grained rock with a waxy to dull or glassy luster that breaks with a smooth, curved conchoidal fracture and sharp edges. It scratches glass easily (hardness 7) and shows no visible grains, layering of mineral crystals, or cleavage. The streak is white.

Colors vary widely; nodules in limestone are common.

Look-alikes include flint (a darker, often glossier variety of chert, essentially the same material), jasper (red/colored, iron-rich chert), and fine-grained quartzite (granular, breaks differently). Distinguishing chert from limestone is easy: chert does not fizz in acid and is far harder. The combination of hardness 7, conchoidal fracture, and lack of acid reaction identifies chert.

Chert's defining historical use was as a toolstone: its hardness and predictable conchoidal fracture let ancient peoples knap it into knives, scrapers, projectile points, and other implements, and certain cherts (like flint) were struck against steel to make fire.

Today chert is used as crushed aggregate and road material, though its hardness can make it abrasive on machinery, and it can cause problems (pop-outs) in concrete. Pure, fine cherts such as novaculite are used as whetstones for sharpening.

Geologically, bedded cherts are important records of deep-sea sedimentation and ancient life, and ancient cherts preserve some of the oldest microfossils on Earth, making them valuable in studies of early life.