Gallery of Chert Rocks and Gemstones

Chert is widespread, but not widely known by the public as a distinct rock type. Chert has four diagnostic features: the waxy luster, a conchoidal (shell-shaped) fracture of the silica mineral chalcedony that composes it, a hardness of seven on the Mohs scale, and a smooth (non-clastic) sedimentary texture. Many types of chert fit into this categorization.

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Flint Nodule

Chert forms in three main settings. When silica is outweighed by carbonate, as in limestone or chalk beds, it may segregate itself in lumps of tough, gray flint. These nodules may be mistaken for fossils.

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Jasper and Agate

Chunk of jasper on a white background. — James St. John/Flickr/CC BY 2.0

The second setting that gives rise to chert is in gently disturbed veins and openings that fill with relatively pure chalcedony. This material is generally white to red and often has a banded appearance. Opaque stone is called jasper and translucent stone is called agate. Both may also be gemstones.

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Gemstone Chert

Selection of chert gemstones on display. — Andrew Alden

Chert's hardness and variety make it a popular gemstone. These polished cabochons, for sale at a rock show, display the charms of jasper (in the middle) and agate (on both sides).

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Bedded Chert

The third setting that gives rise to chert is in deep-sea basins, where the microscopic shells of siliceous plankton, mostly diatoms, accumulate from the surface waters above. This kind of chert is bedded, like many other sedimentary rocks. Thin layers of shale separate the chert beds in this outcrop.

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White Chert

Chert of relatively pure chalcedony is typically white or off-white. Different ingredients and conditions create different colors.

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Red Chert

Red chert owes its color to a small proportion of deep-sea clay, the very finest sediment that settles to the seafloor far from land.

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Brown Chert

Chert may be colored brown by clay minerals, as well as iron oxides. A larger proportion of clay may affect chert's luster, turning it closer to porcelaneous or dull in appearance. At that point, it starts to resemble chocolate.

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Black Chert

Black chard embedded in limestone. — James St. John/Flickr/CC BY 2.0

Organic matter, causing gray and black colors, is common in younger cherts. They may even be source rocks for oil and gas.

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Folded Chert

Folded red chert in a hillside on a sunny day. — Andrew Alden

Chert may remain poorly consolidated for millions of years on the deep seafloor. When this deep-sea chert entered a subduction zone, it got enough heat and pressure to harden it at the same time it was intensely folded.

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Diagenesis

Rock that has gone through chertification process. — James St. John/Flickr/CC BY 2.0

Chert takes a little bit of heat and modest pressure (diagenesis) to lithify. During that process, called chertification, silica may migrate around the rock through veins while the original sedimentary structures are disrupted and erased.

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Jasper

The formation of chert produces an infinite variety of features that appeal to jewelers and lapidarists, who have hundreds of special names for the jasper and agate from different localities. This "poppy jasper" is one example, produced from a California mine that is now closed. Geologists call them all "chert."

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Red Metachert

As chert undergoes metamorphism, its mineralogy doesn't change. It remains a rock made of chalcedony, but its sedimentary features slowly disappear with the distortions of pressure and deformation. Metachert is the name for chert that has been metamorphosed but still looks like chert.

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Metachert Outcrop

In outcrops, metamorphosed chert may retain its original bedding but adopt different colors, like the green of reduced iron, that sedimentary chert never shows.

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Green Metachert

Determining the exact reason this metachert is green would require a study under the petrographic microscope. Several different green minerals may arise through the metamorphism of the impurities in the original chert.

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Variegated Metachert

Piece of multicolored chert on a white background. — James St. John/Flickr/CC BY 2.0

High-grade metamorphism can change the humblest chert into a bewildering riot of mineral colors. At some point, scientific curiosity has to give way to simple pleasure.

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Jasper Pebbles

Jasper rocks of multiple colors. — Andrew Alden

All the attributes of chert strengthen it against erosional wear. You'll see it often as an ingredient of stream gravel, conglomerates and, if you're lucky, as the star character in jasper-pebble beaches, naturally tumbled to its best appearance.