Sedimentary Rocks · common

Chert

Chert is a hard, fine-grained sedimentary rock composed almost entirely of microcrystalline or cryptocrystalline quartz, formed either by direct chemical precipitation of silica or by the accumulation and diagenetic alteration of silica-secreting organisms like radiolarians and diatoms. It breaks with a distinctive conchoidal fracture that produces sharp edges, a property that made it invaluable to prehistoric toolmakers. Chert occurs in nodules within limestone and chalk or as bedded layers, and its color varies widely depending on trace mineral impurities.

Photo: James St. John · Wikimedia Commons · CC BY 2.0

Chert properties at a glance

Type	Sedimentary Rocks
Mohs hardness	6.5–7
Luster	Waxy to dull
Fracture	Conchoidal
Specific gravity	2.6–2.65
Transparency	Translucent to opaque
Common colors	Gray, White, Black, Brown, Red, Green
Rarity	common

Geology & background

Chert forms in marine and lacustrine settings in two main ways: as nodules growing within limestone through diagenetic replacement of calcium carbonate by silica-rich pore fluids, or as bedded deposits accumulating from the skeletal remains of silica-secreting plankton on ancient ocean floors. Notable occurrences include the bedded cherts of California's Franciscan Complex, the flint-rich chalk cliffs of southern England and France, the Arkansas novaculite region, and Precambrian banded iron formations where chert alternates with iron oxide layers. Because it resists weathering better than surrounding limestone or shale, chert often survives as residual nodules and gravel long after the host rock erodes away.

Rockhounds should know that chert's color and banding vary enormously, from gray, brown, and black to red jasper-like varieties colored by iron oxide, and that fossils are sometimes beautifully preserved within it due to the fine grain size that captures microscopic detail. Nodules found in stream gravels and plowed fields are a common and accessible find, and archaeological interest often overlaps with rockhounding since chert was a preferred raw material for stone tools worldwide.

How to identify Chert

Chert is typically dull to waxy in luster, dense and fine-grained enough that individual quartz crystals cannot be seen without magnification, and it always has a white to very pale streak reflecting its quartz composition. Its hardness of about 7 will scratch glass and steel, distinguishing it from softer look-alikes like limestone or chalcedony banding aside, and its characteristic conchoidal fracture produces smooth curved surfaces with sharp edges. Chert is frequently confused with flint, jasper, and chalcedony, all of which are essentially the same silica-based material distinguished mainly by color, banding, and traditional naming rather than true compositional differences.

Compare the specimen's color, luster, crystal habit, grain size, streak, and locality before confirming.
Use multiple photos when possible, including a fresh surface and a close view of texture.

Where Chert is found

Reported from multiple localities worldwide; verify locality against specimen provenance when available.

How much is Chert worth?

Entry-level specimen	$1
Typical collector specimen	$10
Premium specimen	$75+

Value depends on size, crystal quality, color, locality, rarity, damage, and provenance.

Common uses

Field identification
Fossil and depositional environment study
Construction or aggregate when suitable
Education

Interesting facts

Prehistoric peoples on every inhabited continent relied on chert for knapping tools because its conchoidal fracture produces predictably sharp cutting edges.
Bedded chert deposits in ancient ocean basins can preserve exquisitely detailed microfossils of radiolarians that are otherwise nearly impossible to find intact.
Novaculite, a very fine-grained variety of chert from Arkansas, has long been quarried specifically for use as sharpening whetstones.