Hints for Ohio's Flints

This activity will allow you to use your powers of observation to potentially locate your own flint stones next time you find yourself looking down at the gravel in a driveway or at the pebbles and stones piled up along the bank of a stream. A willingness to kneel down and get one's hands a little dirty is the basic requirement for any kind of rock identification activity. Let's dig in!

Rock, Paper, Scissors... Flint!

Things to keep in mind regarding flint:

Flint is typically a dark-colored form of chert, a rock formed from microscopically small bits of quartz
Flint can be easily identified (with practice) by considering basic properties like color, luster, and texture
Flint may be considered one of Ohio's earliest important resources considering its importance in early Native American society

Flint and Steel

Sparks Fly When We're Together

Many of us grow up knowing that flint can be used to make fire but the short video clip here shows how it's actually done (closed captioning is available, turn it on if you desire). As you are watching, be wary of the following things:

What materials are needed besides the flint and steel
What the narrator does to "grow" the flame from the spark

Edged Tools

Cutting to the Chase

Fire making not enough for you? Here's a short time-lapse video showing flint being formed into a large spearhead. Ask yourself this as you're watching: is it practical to try making cutting tools from small, pebble-sized pieces of flint?

Monkey See, Monkey Do

Time to Get Up to Get Down!

Interested? Bored? Ready to get up, go outside and do something? The short video section here is a brief tutorial on sorting flint from other rocks based on color, luster, and texture. Building on what was stated at the beginning of this lesson, most flint in northern Ohio is dark gray in color and it also has a waxy or greasy shine (luster) as well as a smooth texture due to the grains of the rock being microscopic. Geologists describe this texture as cryptocrystalline. With a little practice these properties can usually be sufficient enough to identify flint by sight alone. However, following this activity are other tests you may find useful for confirming if you have found flint.

Mineral Hardness, Acid Reactivity, and Fracture

The following three short videos provide details of some additional tests you may perform in order to identify flint or other types of rocks. Heed the details posted with each video regarding SAFETY!

Flint Hardness

Geologists evaluate mineral hardness using Mohs scale of hardness (scroll down to see it). The scale ranges from 1 to 10, with talc (1) being the softest and diamond (10) being the hardest.

Acid Reactivity

Flint will not react when you put a drop of weak acetic acid (white vinegar) on it. However, calcite-containing rocks such as limestone or marble will react. This test works best if you scratch the surface of a dry sample rock with a nail or screw first. A clue that your sample rock could be flint is that steel is not hard enough to scratch the surface of flint!

Fracture

Quartz, the mineral that makes up flint, has a conchoidal fracture; a freshly broken surface will have a smooth, curved, glass-like texture and form sharp edges. Please be safe if you use a hammer to break rocks, i.e., wear eye protection and make sure you are not hammering near anyone or anything that can be damaged by flying bits of rock.

Additional Resources

Mohs Scale of Hardness

This scale ranks minerals by hardness. Here's an example of how it can be used:

If I have a mineral and I know that it is either gypsum or calcite, I can try scratching its surface with my fingernail.
If my fingernail can scratch a groove in it then I know it is softer than 2.5 on Mohs scale and therefore gypsum, which has a hardness of 2.
I could also try scratching the face of a penny with the mineral.
In this case since gypsum has a hardness of 2 it will not scratch the penny since the penny has a hardness of 3.
Therefore, harder minerals can scratch softer minerals, but softer minerals cannot scratch harder minerals.

The more you try to be gneiss...

...the more you get taken for granite.

Review and Reflection

I hope you've learned a few things about flint today. Keep in mind that the knowledge you gain from identifying just one type of rock or mineral can be used to start learning the identities of other rocks and minerals. If you do manage to find some flint outside then congratulations! Some things to consider:

Which visual properties do you feel are most useful for identifying flint: color, luster, or texture?
Which test do you feel would be most useful for confirming flint identification: hardness, acid reactivity, or fracture?

Quick Quiz

This is meant more for being a means of patting yourself on the back for knowing or learning something new, not as an "oh great, another test I have to pass..." sort of task. Give it a shot - you will be great!

Useful Information from the Web

Mohs scale of hardness: https://geology.com/minerals/mohs-hardness-scale.shtml
Acid reactivity in minerals: https://geology.com/minerals/acid-test.shtml
How flint and steel produce sparks: https://www.quirkyscience.com/sparks-from-steel-and-flint/

About the Author

Prickly Pear*

*The Nuhop Outdoor Education Staff use "nature names" when working with students. The reason for these nature names is twofold: first it gives them a built-in conversation starter with students, and second, it provides them a level of personal privacy when working with over 4000 students in a given year.

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