Chert – the birthstone of our species

Few types of stone have as long lasting and intimate relationship with our species as do those of the chert family.  Humans have been using this hard, glassy stone continuously to make tools since the time of Homo habilis, some 1.5-2 million years ago.  Our neolithic ancestors mined chert using fire to crack the stone (see video to see how it was done), at least 33,000 years ago at the Nazlet Khater site in Egypt chert was extracted from subterranean mines,  and flint (a type of chert) was used before we had matches to make fire, today we use crushed chert as the abrasive on some sandpapers, and to extract exquisitely detailed micro-fossils from the distant past.  When I worked as an archaeologist near Santa Barbara most of the projectile points, stone awls, and cutting tools we found were made from chert.

What is this “chert” we have been using so assiduously for the last 2 million years?

Chert is a microcrystalline stone made of silicon dioxide (SiO2) with a cryptocrystalline structure (crystals so fine that they are difficult to see even under a microscope) that lacks cleavage planes.  One of the most useful, for us, aspects of cryptocrystalline materials such as chert is that when they are struck they shatter in a predictable conical manner (conchoidal fracturing).  Obsidian and plate glass share this characteristic, making them and chert excellent for making extremely sharp stone tools.  Chert is more common than obsidian, but still rare enough that it was traded over great distances.  Another beneficial aspect of chert is that it is extremely hard, raking a 7 on the Mohs scale.

A number of well known minerals fall into the chert category: flint, jasper, radiolarite, chalcedony, agate,  and onyx are all types of chert, each with specific characteristics that give them enough difference from each other to warrant specific names.  The famous (and expensive) sharpening stones from Arkansas are made from novaculite, a porous, metamorphosed chert that makes an excellent abrasive.  Flints tend to be high quality cherts that are found specifically in chalk or limestone; these are deposited diagenically (via silicon replacement).  Chalcedony, agate, and onyx are a nested subset of minerals with chalcedony, a fibrous form of chert, being the parent of the group.  Jasper is usually found in association with volcanic activity and is sometimes considered to be under the chalcedony subset.  Jasper, agate, and onyx are popular semi-precious stones used for jewelry and sometimes intricately carved.

Red jasper cameo of Medusa by Benedetto Pistrucci (source)

What got me started thinking about cherts once more was a short day-hike I took with a friend in the Marin Headlands, those steep sided hills to the north of San Francisco that are so often obscured by the thick maritime fog.

Marin Headlands and the Golden Gate bridge partially hidden by fog

Marin Headlands and the Golden Gate bridge partially hidden by fog

The Northern California coast has a complex geology and is undergoing a number of divergent changes simultaneously.  This is an emergent shoreline (Geology of Northern California chapter 10, page 37), a place where the land is slowly rising in elevation.  Rising lands often suffer from high rates of erosion and the California coast has an even more drastic set of factors contributing to the erosion rare than merely rising land.  The bedrock is fractured by many faults, weakening the stone, earthquakes (most extremely minor) shake rubble loose periodically, and both wind and water eat away at the ocean facing slopes.  In addition, the sea level has risen several hundred feet since the last glaciation twelve thousand years ago, and the surf is greedily pounding on the hills, tearing parts of them away.

If you watched the video you probably noticed that the chert they were mining peeled off in plates, indeed that the whole formation was made up of sheets of stone layered atop each other like pastry dough.  About 50% of rock in the Marin Headlands is chert and has a similar texture.

Radiolarin ribbon chert cliff showing soft folds and a sharp fault-line cut

Radiolarite chert cliff showing soft folds and a sharp fault-line cut

This is ribbon chert, more formally known as radiolarite chert.  It gets the latter name because it is a biogenic stone made from the semi-gelled skeletons of radiolaria, a type of plankton that builds a silica based support structure.  These this rock was laid down over a 100 million year span beginning 200 million years ago and is filled with tiny fossils of the radiolaria.  Supposedly some of these are large enough to see with a standard hand-lens.

The folding tells us something interesting.  The folds in the above photo are smooth, meaning that this probably slumped slowly while it was still ductile.  Portions of the cliff have sharp folds where the rock broke, indicating that those deformations most likely happened more rapidly and after the stone had lost much of its ductility.

Red and greenish/blue indicate iron, either in oxodised or reduced form

Red and greenish/blue indicate iron, either in oxidised or reduced form

Much of the chert here is red, but there are many patches of vibrant blue-green and aqua as well.  The colors in chert indicate trace amounts of other minerals.  The red and the lovely greenish-blue are both indicators of iron, the red indicating that the iron has oxidised, the blue-green that it has been reduced (had the oxygen removed from it).  This is similar to the mottled gleying that one sees in wetland clay soils.  I am particularly fond of the blue and green colors in chert, perhaps because they are a bit more rare than the red.

Close-up of blue/green chert bands

Close-up of blue/green chert bands

The hardness of the chert leads to beaches with an interesting texture of sand, more like tiny glossy pebbles than the standard sand.

Chert sand

Chert sand

The combination of colors on the cliffs, beach, sky, and ocean make for a nice combination as well.

Tennessee Valley beach in the Marin Headlands

Tennessee Valley beach in the Marin Headlands

We have been using chert for nearly as long as we have been using tools, close to 2 million years now with no sign of slowing down.  If our species had a birthstone it would probably be chert.


7 comments on “Chert – the birthstone of our species

  1. kevin cosgrove says:

    thanks for your blog… saw lots of fractured chert but no conchoidal fractures ? I agree, chert is a wonderful material.

    • EarthKnight says:

      Hi Kevin, glad you like the blog,

      To produce conchoidal fractures takes a specific kind of strike and a chert with few internal fractures or inclusions. The chert in these specific photos has been uplifted, folded, and squeezed so much that it is full of tiny cracks that inhibit the formation of a good, smooth conchoidal fracture.

      This photo from Flikr shows some of the conchoidal fractures on the block of what appears to be Franciscan Formation chert in the lower left:

      • There are many types of “chert”. Glassy chert comes from volcanic ash. The sort of shaly mudstone shown here comes from seafloor ooze perhaps with radiolaria. No amount of faulting will stop conchoidal fracture. Agate comes from volcanism with the silica from a melt. The different materials making these different silica rock types accounts for their different physical features. You cannot dissolve silica and have perfectly preserved radiolaria any more than you can have your cake and eat it too. Ash causes radiolaria blooms, which require the microglass, high surface area of ash to utilize. Sand at the beach cannot make a radiolaria bloom. As the ash falls into the shelf sea floor, the bloom dies out, and they fall to the floor as well. Without the ash events, the ocean has no silica to allow radiolaria to live. This is why the cherts have nicely preserved radiolaria, if round quartz globs with zero ornamentation in cherts can be proven to be altered radiolaria, instead of say, cristobalite microspheres, which also form after ash fall events. All cherts beds are found with volcanic ash because coastal upwelling does not make radiolaria blooms. Diatomite sites of the Western U.S. have zero agates associated with them because diatoms do not alter to silica to make agate. Agates are found in volcanic rocks and some cherts subjected to major ash fall events such as those deposits to the East of Yellowstone, buried in 300 ft of ash. Mud with silica is not chert. This is why the term porcelanite is used to describe those rocks, which contain too much foreign matter to be coherent. No seafloor material itself can make chert or agate or jasper because none can form with feldspar, a common weathering mineral found in those sediments.

  2. The green coloration is from glauconite, a weathering product of volcanic ash. Its color comes from reducing conditions, but is not specifically iron, but an iron-rich silicate. In continental conditions, reducing condition transitional boundary makes celadonite, a very similar clay.

    • EarthKnight says:

      Hi Donald,

      Thanks for the detailed reply and the good information in it. My research and personal experience into the subject are a little at odds with what you posted vis-a-vis the strict necessity of volcanic ash falls and the lack of agates on California beaches. Agates are often found in conjunction with vulcanism, true, and there are a lot of relict volcanic remnants on the California coast, but agates are also found in conjunction with fossils, and in a few other situations as well.

      The necessity for volcanic ash in the formation of chert is an interesting idea and one that appears to being bandied back and forth still.

      The comment about lack of conchoidal fractures in some cherts was more from the perspective of tool making where heavily fractured cherts where the formation of a flake of useful size is inhibited by the breaks in the rock structure. There is still definitely conchoidal fracturing, but it may be at such a small level that you need a magnifying glass to see it and is useless for tool making.

  3. morenich says:

    Wow, what an awesome blog, thanks for posting all of this information with sound science and correct terminology!

    One of the photos of the Radiolarite chert cliff looks like it was taken off of Conzulman road. I had been riding and walking by these cliffs in awe as they were doing the road-widening project there. Now that I have this new information about the geology, I can’t wait to return to ponder the cliffs up close and gaze at the beautiful structures in the rock.

    I’m elated to find another person who is immersed in the natural world, inquisitive and bound to leave the world a better place.

    Thank you Neahga.

    • EarthKnight says:

      Thank you Mike.

      None of the photos in this post were taken from Conzulman Rd, but the formation is the same. There are some lovely exposed outcrops right next to the overlook parking spaces. People think I’m odd because they’ll be looking at the view and I’m over peering at the hillside.

      The photos of the chert were all taken at the beach at the end of Tennessee Valley.

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