Chert Gallery

01
of 17

4 Features of Chert: Luster, Fracture, Hardness, Texture

Chert diagnostic features
Chert Gallery Specimen from the Mojave Desert. Photo (c) 2012 Andrew Alden, licensed to About.com (fair use policy)

Chert is widespread but not widely known by the public as a distinct rock type. It helps to see examples. That's what this gallery is for. To learn more of the geological details, see About Chert.

Chert has four diagnostic features: the waxy luster and conchoidal (shell-shaped) fracture of the silica mineral chalcedony that composes it, a hardness of 7 on the Mohs scale, and a smooth (non-clastic) sedimentary texture.

02
of 17

Flint Nodule

Flint nodule
Chert Gallery. Photo submitted by an About.com Geology reader (fair use policy)

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.

03
of 17

Jasper and Agate

Jasper specimen
Chert Gallery Jasper from Lompoc, California. Photo courtesy Phil Vogel; all rights reserved

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 be gemstones.

04
of 17

Gemstone Chert

Gemstones on display
Chert Gallery. Photo (c) 2011 Andrew Alden, licensed to About.com (fair use policy)

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).

05
of 17

Bedded Chert

Bedded chert outcrop
Chert Gallery Outcrop of Claremont Formation, Oakland, California. Photo (c) 2012 Andrew Alden, licensed to About.com (fair use policy)

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.

06
of 17

White Chert

Basic white chert
Chert Gallery Chert in the Berkeley Hills. Photo (c) 2007 Andrew Alden, licensed to About.com (fair use policy)

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

07
of 17

Red Chert

From deep-sea red clay
Chert Gallery Chert of the Franciscan Complex, coastal California. Photo (c) 2012 Andrew Alden, licensed to About.com (fair use policy)

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.

08
of 17

Brown Chert

Brown chert
Chert Gallery. Photo (c) 2012 Andrew Alden, licensed to About.com (fair use policy)

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. At that point it starts to resemble chocolate.

09
of 17

Black Chert

Black organic-rich chert
Chert Gallery Claremont Formation at Alum Rock Park, San Jose, California. Photo (c) 2011 Andrew Alden, licensed to About.com (fair use policy)

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

10
of 17

Folded Chert

Folded red chert
Chert Gallery Radiolarian chert of the Marin Headlands, California. Photo (c) 2006 Andrew Alden, licensed to About.com (fair use policy)

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.

11
of 17

Chert Diagenesis

Chert in the process of chertification
Chert Gallery Chert boulder from Tucson, Arizona. Photo courtesy Eric Price; all rights reserved

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.

12
of 17

Picture Jasper

Poppy jasper rough
Chert Gallery. Photo (c) 2009 Andrew Alden, licensed to About.com (fair use policy)

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."

13
of 17

Red Metachert

Red metachert
Chert Gallery Franciscan metachert, Oakland, California. Photo (c) 2005 Andrew Alden, licensed to About.com (fair use policy)

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 cherty.

14
of 17

Metachert Outcrop

Metachert in outcrop
Chert Gallery Mountain View Cemetery, Oakland, California. Photo (c) 2012 Andrew Alden, licensed to About.com (fair use policy)

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

15
of 17

Green Metachert

Olive-green metachert
Chert Gallery. Photo (c) 2005 Andrew Alden, licensed to About.com (fair use policy)

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

16
of 17

Variegated Metachert

A beautiful chert
Chert Gallery Available in a wallpaper version. Photo (c) 2005 Andrew Alden, licensed to About.com (fair use policy)

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. This image is available in a wallpaper version.

17
of 17

Jasper Pebbles

Jasper beach
Chert Gallery Gravel of Rodeo Beach, California. Photo (c) 2006 Andrew Alden, licensed to About.com (fair use policy)

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.

Format
mla apa chicago
Your Citation
Alden, Andrew. "Chert Gallery." ThoughtCo, Jan. 13, 2017, thoughtco.com/pictures-of-chert-4122739. Alden, Andrew. (2017, January 13). Chert Gallery. Retrieved from https://www.thoughtco.com/pictures-of-chert-4122739 Alden, Andrew. "Chert Gallery." ThoughtCo. https://www.thoughtco.com/pictures-of-chert-4122739 (accessed January 20, 2018).