Flint is a very popular rock with collectors, myself included. It can be found in locations all over the world, sometimes even as gem-quality material that is highly sough-after by enthusiasts. Despite its popularity, there are a lot of misconceptions about flint. I thought it would be helpful to go over what flint is, how to identify it, and how it differs from closely related rock types.
Flint is a high-quality variety of chert, a biochemical sedimentary rock made of cryptocrystalline quartz. It is very hard, with characteristic conchoidal fracturing that makes it exceptionally useful for the creation of tools like arrowheads. Flint comes in many colors but is usually dark gray or black.
While flint is a clearly defined rock type, it can still be difficult to know whether a rock you’ve found is flint, especially when you consider there are so many similar-looking rock types out there. I’ll walk you through how to identify flint, how it forms, and where it can be found.
What Does Flint Look Like?
It can sometimes be difficult to know if you’ve found a piece of flint because one piece often looks very different from another, especially if they are from different localities. But usually, there are general attributes that practically flints share in common.
In general, flint looks like a mass of sediment with relatively smooth, waxy-looking surfaces. It displays conchoidal fracturing resulting in very sharp edges. Flint can be almost any color but is usually grey, black, brown, or green, and often has a whiteish ‘rind’. It is usually opaque, but sometimes translucent.
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While all flint fits this general description, there is still quite a bit of variability from specimen to specimen. Because there is so much variation, it is best to focus on the attributes they all have in common when trying to visually identify flint.
Flint is made from microscopic quartz crystals, too small to be seen even under low-powered magnification. This contributes to flint’s overall smooth, dull, and waxy appearance. There are no large crystals to make the rock’s surface look or feel rough. It can sometimes feel like you’re holding a piece of hardened wax in your hand.
Perhaps the most defining characteristic of flint is its sharp edges and conchoidal fractures. Flint is known for its tendency to break in convex ‘scoop’ shapes that converge into very sharp edges. It is this tendency that made it so useful for primitive tools like knives and arrowheads.
Flint comes in many colors, but most of the time it is dark gray, black, brown, or greenish. These colors are common in many rocks, but it can sometimes be useful to note the color when trying to distinguish flint from other varieties of chert.
Color is Driven by Impurities
Flint is, by definition, made almost entirely of cryptocrystalline quartz – the mineral version of SiO2. Pure quartz is almost completely transparent, so if flint were made of pure cryptocrystalline quartz you’d be able to see through it like glass.
But we know that’s not the case with almost any piece of flint found in nature. This is because flint is laden with impurities in the form of trace elements, and each of these impurities contributes some coloration.
Some of the most common impurities in flint are iron, aluminum, manganese, sodium, and calcium. Organic material and water are also frequently included in the fabric of flint. Each of these impurities has a distinct color associated with it. For instance, enough iron will turn flint red, as is often seen in the famous Ohio Flint. Similarly, organic material will turn it dark gray or black.
Pro Tip: To get a better look at the crystals in your rocks and help with identification I highly recommend picking up a good geologist’s hand lens.
Translucent or Opaque?
You might be wondering why some varieties of flint are opaque while others are translucent. The answer is actually twofold, and they work in conjunction with one another.
The amount and type of impurities in flint are significant contributors to how translucent it is. If there are a lot of trace elements present, especially dark ones like iron and organics, then the rock is much more likely to be opaque. Conversely, a smaller amount of light-colored impurities are likely to produce a transparent variety.
But impurities aren’t the only (or even the most significant) contributor to flint’s diaphaneity (how light passes through a rock).
Flint is made of microcrystalline quartz, but not all microcrystalline quartz forms the same way. All of the tiny crystals are too small to see with the naked eye, but sometimes they are blocky and other times they are more elongated or fibrous.
Translucent varieties of flint are made of more fibrous microcrystals, while opaque varieties are made of blockier crystals. This small change has a drastic impact on how easily light passes through a specimen.
How to Identify Flint
By now you might be thinking that flint must be difficult to identify, especially with so many different-looking variations. But, in most cases, flint is pretty easy to recognize if you know what to look for. As with any rock, it is important to take a systematic approach when identifying flint and its variants.
Flint usually has a waxy luster, with conchoidal fracturing that results in sharp edges. All flint is very hard and cannot be scratched by steel. Most varieties are opaque, but some are translucent. Flint usually forms as nodules with gray, black, or brown interiors, and often has a whiteish exterior ‘rind’.
As I mentioned above, conchoidal fracturing is a very good diagnostic characteristic when trying to identify flint. It looks like someone has scooped out part of the rock with a spoon, resulting in smooth, convex depressions on the surface of the rock. These shapes are usually lined with edges that can be very sharp, which is why flint and chert were so desirable for making tools like arrowheads and primitive knives.
Your rock may not display clear conchoidal fracturing. There are also some other rock types (like obsidian) that fracture conchoidally but aren’t flint or chert. Therefore, some additional tests and observations are often useful.
A simple hardness test is one of the best ways to check for flint and other types of chert. Since it is almost entirely made of quartz, chert is exceptionally hard – registering a 7 on the Mohs hardness scale. This is harder than steel! Use a steel nail and try to scratch your rock. If it doesn’t leave a scratch, then your rock is very hard (most likely quartz) and could potentially be flint.
Since flint is made of silica (quartz) and not calcite or dolomite it will not react with acid. If you’ve already performed a hardness test and your rock is harder than steel, it is not even necessary to conduct this test since both calcite and dolomite are much softer than steel.
In summary, a rock must meet all of these requirements to be considered flint:
- Chemical or Biochemical – A sedimentary rock formed from the solidification of microscopic silica particles
- Very Fine-grained – Individual grains are too small to see, even with a hand lens
- Very Hard – Cannot be scratched by steel (7 on the Mohs hardness scale)
These characteristics are common to all varieties of chert, including flint. Beyond that, the many varieties of chert need to be identified separately. In general, hobbyists usually consider flint to be an opaque and very high-quality variety of chert. It takes a polish well, and is often used in lapidary work for jewelry and other decoration.
If your rock meets all of those criteria then it is very likely flint, or at least something very closely related. There are, however, a few closely related rocks that people sometimes confuse for sandstone.
- Common Chert – Very similar and closely related to flint, but usually off-white or light brown and generally lower quality than flint.
- Obsidian – Might be confused for flint or chert due to smooth texture and conchoidal fracturing. Softer than flint, with a glassier texture.
- Micrite – Similar texture and color to flint and also sedimentary in origin, but much softer than chert and it reacts with acid.
- Crystalline Quartz – Same hardness as flint, but is one large, single crystal with defined, flat faces. Usually translucent or transparent.
Tip: This article is part of my sedimentary rock identification series. To read more about how to identify all igneous rocks, check out my article here.
Flint vs Chert
It can often be very difficult to label a rock as ‘flint’ or ‘chert’. The line between the two is blurry and is can be debated from specimen to specimen. All flint is chert, but not all chert is flint. Other varieties of chert include agate, chalcedony, and jasper.
It is commonly accepted that flint is usually a high-quality, opaque variety of chert that is usually gray, black, or brown in color, although colors like green and red are also possible. Agate and chalcedony are distinguished by their translucency and overall light coloration, while jasper is usually more colorful (red is most common) and doesn’t display the strong conchoidal fracture tendency that jasper does.
What Is Flint Made Of?
We have already seemingly answered this question earlier in this article – flint is made of cryptocrystalline quartz, right? But the real answer is a little more involved than that, and is actually much more interesting than it seems at first glance.
In general, flint is made of cryptocrystalline or microcrystalline quartz, the mineral version of SiO2. These particles are often microfossils that are fused together to form a solid mass. Varieties of flint also contain impurities in the form of trace elements like aluminum and iron, as well as organics or water.
The microscopic quartz crystals are usually the hard, petrified remains of organic sediment that settle on the ocean floor. Certain creatures in the sea, both past and present, leave behind tiny siliceous shells when they die. These remains are the building blocks of biochemical flint, which is thought to be by far the most common way it forms.
When flint forms biochemically, it is technically mostly made of very small fossils. These fossils are too small to see without high-powered magnification, but they are fossils nonetheless. Organisms like diatoms and radiolarians secrete silica-based shells instead of calcite like most of the seashells we’re familiar with.
Flint is usually found in nodules contained with layers of limestone, chalk, or marl. The process by which this occurs is still not well understood, but may be related to burrowing sea life shortly after deposition. These nodules usually occur at the boundaries between these layers of sediment.
Large fossils like seashells and coral are possible in flint, but are relatively rare. If they are present, it is usually in biochemical flint. Visible fossils in flint aren’t as common as they are in other sedimentary rock types like limestone and sandstone.
Where Is Flint Found?
By now you are probably asking yourself where you can find some flint of your own. Or maybe you’ve already found a rock that you suspect might be flint and you’re wondering if it occurs in your area. Flint is a fairly common rock type and it occurs in locations all around the globe, but it helps to know the general settings in which it can be found.
In general, flint is found interbedded with limestone and chalk, usually as nodules but sometimes as continuous beds. Many large flint beds are found in Jurassic and Cretaceous aged formations all around the world. In the U.S., the most famous flint location is Flint Ridge in Ohio.
Biochemical flint, formed from the collection and solidification of tiny silica fossils at the bottom of the ocean, is most likely to be found in areas with a geologic history of low-energy marine environments like deep ocean water. These same environments are prone to creating chalk, so it is common to find flint beds associated with chalk.
You can look for flint-bearing formations near you using this excellent interactive map from the USGS. I have a video about how to use this tool in my Practical Rock Identification System, plus even more information on how to identify flint and other rocks.
Usually, you’ll find flint formations near other sedimentary rocks like limestone, chalk, shale, and marl. These rocks tend to form in cycles and groups related to the relative water levels at the time of deposition.
You don’t necessarily need to find a flint-bearing outcrop in order to find flint, though. Flint is so hard and durable that it can be transported a great distance while remaining relatively intact. You can sometimes find flint nodules in river and stream beds downriver from flint-bearing rock formations.
How Does Flint Form?
We’ve learned all about what flint looks like, what it is composed of, and generally where it’s found, but I have only briefly touched on how it’s actually formed. The creation of flint is not all that straightforward, and is still not all that well understood by geologists.
In general, flint forms when large quantities of microscopic silica crystals are infused into sedimentary rock formations (typically limestone, chalk, and marl) during their formation. These silica crystals are sometimes
Most flint forms when large quantities of microscopic skeletons made of quartz fall to the ocean bottom amidst other sediments that eventually form chalk and limestone. Over time, the silica (quartz) precipitates and forms into nodules or cohesive layers interbedded in the other sediments before hardening.
The process is very complicated and is still hotly debated amongst geologists. It is possible (and even likely) that many of the theories are correct and that flint can actually form in a variety of ways.
One aspect of the formation of flint that seems to be generally accepted is that when the organisms (mostly radiolarians and diatoms) die off, their skeletal remains are originally made of opal, which is a hydrated mineraloid version of silica. Over time, that opal gradually converts to the microcrystalline quartz that makes up flint.
What Is Flint Used For?
Flint has a long history of practical use all around the world, and continues to be popular for many reasons even today.
Flint has historically been used for practical tools like knives and arrowheads due to its conchoidal fractures and sharp edges. It has also long been used as a primitive means to start fires when struck against steel. High-quality varieties flint remain popular materials for lapidary work and jewelry.
You don’t usually see flint used for construction or building purposes. It doesn’t usually occur in large enough quantities to facilitate building stones, and there are problems with its chemistry and tendency to fracture that make it impractical to use even as aggregate or filler material.
This article is part of my rock identification series. To learn more about identifying rocks, check out my full in-depth guide here.