Chert Rock: What is Chert?

Chert is a sedimentary rock that is typically found in nodules in surrounding limestone.   Chert rock is mainly comprised of microcrystalline crystals of quartz.  There are some formations of chert rock that can be excluded of biological remains, but typically they are present in this rock.

The remains of open sea life find their way to the bottom of the ocean floor.  This is typically in the form of skeletal microscopic organisms.  This type of sediment has been given the name biogenic pelagic sediment, or siliceous ooze, and will typically be found in chert. 

Identifying Chert

Chert can be identified by how chert is broken, its waxy luster, and lighter colors.  Chert breaks in nice conchoidal pieces that can have sharp edges.  The waxy luster of chert is quite unique and can help identify the mineral.

Due to its silica based crystalline components, chert can appear very smooth with a slight waxy, shiny look to it.  This is a similar look in what you might see with obsidian.

I have a great article on obsidian if you are looking for a reference on this mineral. “All About Obsidian“

The Formation of Chert

Chert will form in carbonate type rocks, whish is a type of sedimentary rock, as nodules or larger masses.  As the sediment that can make up greensand, chalk, dolomite, and limestone get buried deep within the earth over time, these pockets of chert will form as well in the immense pressure and heat.

Microcrystals of silicon dioxide play an important role in the creation of chert.   Dissolved silica in moving ground water made their way in and around these concentrations of chert sediment to crystalize.  Much like what happens with limestone and chalk.

This crystallization of course happened after years of the chert materials being pushed down under the earth.  The pressure and heat crystalized the silica along with the chert sediment materials, siliceous ooze, to give use chert rock.

Why Chert Nodules?

A chert nodule is a small and irregularly rounded knot, lump, or mass in surrounding sedimentary rock.  Having a contrasting chemical composition will separate the chert sediment contents from the limestone sediment contents, or host sediment.

Differences Between Chert and Flint

The differences between Chert vs Flint are not too many.  Flint is actually a variety of chert.  The main distinction is that chert will typically form in nodules in limestone. Flint will similarly form in the same way, but forms within chalk or marl. 

The basic physical properties of chert and flint are not too far off from each other in their composition.  Many scientists run along the rule of distinction between the two in how they were formed.  Others don’t like that and would rather the two be categorized on what they are.  This would basically lump chert, flint, and even jasper into the same accord. 

Some scientists will use chert in more of a general respect, and then use flint as a darker type of the rock.  These same scientists may also distinguish chert as larger bodies of rock, where flint will be more nodular.

Chert and Sparks

As a point of reference, as many do wonder this question, chert and flint are both capable of making a spark when struck against steel.  Again, both do have similar compositions.

A particular TV show series involving a pawn shop had a customer bring in an 18^th century revolutionary rifle called the Flintlock rifle.  The concept of this rifle was there was a piece of flint on the action hammer.  When this hammer was pulled back and then released by the trigger, it would strike steal creating a spark.  This would in turn ignite the gunpowder causing the explosion that would send the bullet down the barrel.  This gun is very rare to obtain and had a high price tag on it.

Uses of Chert

Chert does not seem to have many uses by today’s standards.  In the past chert was used for making weapons, tools, and other uses that required a sharp edge or a spark.  Some use chert as a construction aggregate where it helps in road surfacing applications.

Chert can be very beautiful in its color and look.  Often people will purchase chert as a decorative addition to their yards as landscaping rocks.  Many will purchase chert at rock and mineral shows as specimens due to the same beautiful look.

Some may use chert in building applications or bridges, though you will not see it used as often as something like marble, granite, diorite, or andesite.  This could be due to the way it can break in its conchoidal fashion by my thoughts.

Colors of Chert

Chert can come in quite an array of colors.  White to black, browns, light greens, dark greens, oranges, reds, and most commonly gray, are the colors chert will take on.

Colors in chert are often due to other impurities that are available in the rock including the very sediment itself. 

With many other rocks, or minerals for that matter, the presence of iron oxides can be a producer of reds and oranges.  Organic material will commonly produce more of the grays, or even black, when in greater amounts.

Some distinguish the darker forms of chert as flint and other lighter colors as chert.  This is also considered a potential difference between the two, based on color, as mentioned earlier.

Many Varieties of Chert

There are several varieties of chert.  Some of the more popular varieties of chert include; Agate, Chalcedony, Flint, Jasper, Magadi chert, Mozarkite, Onyx, Opal, Porcelanite, Siliceous sinter, Radiolarite, and Tripolitic chert.

All these varieties consist of having silica in one way or another, and can be considered close and similar to chert in their chemical compositions.

For reference on a couple of these minerals I have some good articles on Agate and Opal. A couple of favorite minerals for many. “All about Agate” and “All about Opal“

Where is Chert found?

Chert can be found in many locations throughout the world.  Chert can be found in oceans and on beaches.  It will be found above the surface in dry locations or mountains as it was pushed up to the surface long ago.

A notable location is the Ouachita Mountains in Arkansas USA.  Oklahoma, Texas, and South Carolina are some other notable places in the US.  Most states in the US have some location where chert can be found. 

Outside of the US will likely have an abundance of chert as well, though finding these places so as to note here, was not as prominent.