OK
1dave , what's the difference between chert and flint? Thanks.
Thanks for the question. I had to dig a bit. Flint is a
variety of chert found in chalk and limestone.
en.wikipedia.org/wiki/Cherten.wikipedia.org/wiki/Flint
Best of all, Andrew Alden: a geologist who writes extensively about all aspects of geology, and leads research expeditions for professional organizations, commented March 28, 2019. He presented
16 photos of varieties of CHERT.Go to the site to see the photos.
www.thoughtco.com/pictures-of-chert-4122739:max_bytes(150000):strip_icc()/collection-167170_1920-9f98bb68773843e9b5a0ef6d6eed1ee9.jpg)
Chert is widespread, but not widely known by the public as a distinct rock type.
Chert has four diagnostic features:
1. the waxy luster,
2. a conchoidal (shell-shaped) fracture of the silica mineral chalcedony that composes it,
3. a hardness of seven on the Mohs scale, and
4. a smooth (non-clastic) sedimentary texture.
{NOTE from
1dave :
Clastic rocks are formed from the eroded parts of other rocks.clastic rocks made of sand, gravel etc.
Non clastic rocks have precipitated chemically from water:
Precipitated from high pH (basic) - carbonate rocks, composed chiefly of calcium carbonate, as the shells of other organisms; or low pH (acidic) - silicates - chert, chalcedony, agate, jasper.}
Many types of chert fit into this categorization.
01 of 16 - Flint Nodule
Flint nodule on a plain surface.
James St. John/Wikimedia Commons/CC BY 2.0
Chert forms in three main settings.
First 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.
02 of 16 - 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.
03 of 16 - 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).
04 of 16 - Bedded Chert
Bedded chert outcrop on a sunny day.
Andrew Alden
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.
05 of 16 - White Chert
Chunk of white chert among other rocks.
James St. John/Flickr/CC BY 2.0
Chert of relatively pure chalcedony is typically white or off-white. Different ingredients and conditions create different colors.
06 of 16 - Red Chert
Red chert showing striations and color bands.
James St. John/Flickr/CC BY 2.0
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.
07 of 16 - Brown Chert
Brown chert next to a coin for scale.
Andrew Alden
Chert may be colored brown by clay minerals, as well by 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.
08 of 16 - 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.
09 of 16 - 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.
10 of 16 - 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.
11 of 16 - Jasper
Jasper gemstone against a plain background.
James St. John/Flickr/CC BY 2.0
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."
12 of 16 - Red Metachert
Red metachert rock formation.
Andrew Alden
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.
13 of 16 - Metachert Outcrop
Metachert outcrop on a sunny day.
Andrew Alden
In outcrops, metamorphosed chert may retain its original bedding but adopt different colors, like the green of reduced iron, that sedimentary chert never shows.
14 of 16 - Green Metachert
Olive-green metachert found in nature.
Andrew Alden
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.
15 of 16 - 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.
16 of 16 - 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.
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