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Environment Environmental Issues

What is Chalk?

Mary McMahon

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Updated: Jun 04, 2024

Chalk is a form of limestone that is distinguished by its relative purity and close, fine grain. Humans have been using it for a variety of tasks for centuries, taking advantage of its properties for things like writing on blackboards, marking out fabric patterns for cutting, and delineating game fields in sports like tennis and American football. This mineral is extremely abundant, and it is widely quarried all over the world.

The chemical formula for chalk is CaCO3. Its primary ingredient is ancient fossilized sea organisms, which were deposited and subjected to high pressure over centuries, so deposits often occur in areas that were once underwater. Rocks that form through the deposition of sediments and other materials are known as sedimentary rocks; chalk is probably one of the most well known examples of this type of rock, thanks to its common use. Impurities like quartz, feldspar, and silica are often found in the deposits.

Although people often think of chalk as a very soft mineral, it is in fact remarkably resistant to weathering. This trait led to the formation of the famous White Cliffs of Dover, as the elements wore away at other mineral materials in the large limestone deposit that streaks across much of Europe, leaving imposing deposits of chalk in the form of cliffs and sea stacks behind. The mineral is also very porous, and it tends to store groundwater extremely well, releasing it during drier periods.

The natural mineral can range in color from yellow to gray to white, depending on the impurities it contains. Depending on what it is going to be used for, it may be treated to remove impurities and refined so that it turns white. It can also be dyed with various coloring agents to produce things like sidewalk chalk.

Most classroom chalk today is not actually made from limestone, but rather from various other mineral composites. The powdered material that was once used to mark out sports fields has also since been replaced by various other minerals or paint. It is still possible to find a few commercial products made from chalk, and many natural history museums keep it on display since it is a mineral of interest to many people.

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By Mary McMahon

Ever since she began contributing to the site several years ago, Mary has embraced the exciting challenge of being a All Things Nature researcher and writer. Mary has a liberal arts degree from Goddard College and spends her free time reading, cooking, and exploring the great outdoors.

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Discussion Comments

By seag47 — On Nov 19, 2012

Be aware that if you give your child chalk as a gift, you may have to sacrifice your garage floor for awhile. I gave my niece a box of colored chalk for her fourth birthday while she was staying with me for a week, and I had to give up my parking spot so that she could work on her art project!

By cloudel — On Nov 18, 2012

I remember finding a thick piece of yellow chalk in my friend's attic. We used that chalk to draw out hopscotch boxes on the sidewalk, and we had so much fun!

My friend's grandmother had been a schoolteacher, and she had recently passed away. The family had gotten a lot of her old stuff, and they just stored it all in the attic. We had been curious about what was up there, and when we found that chunk of chalk, it was like gold to us!

We didn't have a lot of toys, because our parents were poor. It's amazing how much one piece of chalk lit up our lives!

By lighth0se33 — On Nov 17, 2012

@claire24 – If the chalk is dustless, it isn't bad for kids at all. The dust is what causes respiratory problems for some kids, especially those prone to asthma attacks.

Also, if you have a computer in the room, the dust from the chalk can accumulate in it and cause it to overheat. It's safer just to use dustless chalk or a marker on an erasable board.

By DylanB — On Nov 17, 2012

@golf07 – That chalk dust was really irritating to me, because I have issues with allergies and asthma. If I had to erase something that was on the blackboard, I would begin wheezing and start coughing.

Any kind of irritant that floats around in the air like that can get into your lungs and cause problems. Chalk dust was so fine that I easily inhaled it without trying to, and I would suffer for the rest of the period. This is why I never sat near the blackboard.

By ZsaZsa56 — On Sep 11, 2012

Lots of people think that chalk is just for kids but it is actually one of the most interesting and flexible artistic materials. I have seen artists do amazing things with chalk. Better yet, I have seen amateurs do amazing things too.

And what is the best part of all is that you can do art on the street and not have it be permanent. I always loved that about chalk. It can make such an impact but also be completely temporary.

By LisaLou — On Aug 26, 2012

I bought a box of Crayola chalk that was anti-dust and really couldn't tell that much of a difference. It had been awhile since I used a box of regular chalk though, so maybe I just forgot how much dust there was before.

My kids like to use the Crayola drawing chalk. They don't use this on chalkboards, but draw with it on paper. Once they get too old to use crayons, the colored chalk is a fun alternative for them.

By John57 — On Aug 25, 2012

You very seldom see a chalkboard in a classroom setting today. Many of them have been replaced with whiteboards. These are certainly less messy, but for me there is something nostalgic about a chalkboard.

They do make dustless chalk which does cut down quite a bit on the mess. My friends and I always enjoyed the job of cleaning the erasers. We got to leave the classroom and go outside for awhile.

By Mykol — On Aug 25, 2012

My kids love sidewalk chalk and can stay busy for quite awhile if they have a bucket full of bright pieces of chalk.

They are always disappointed when the rain erases their artwork, but this just gives them reason to be creative all over again.

I find it hard to resist just watching them when they are playing with the sidewalk chalk. There is something relaxing and fun about making designs and patterns on the sidewalk with big, brightly colored pieces of chalk.

By golf07 — On Aug 24, 2012

When I was in elementary school, our teachers wrote on the blackboard with pieces of chalk. As a young kid, I never really thought about where chalk came from. I just hoped it was never my turn to go to the front of the class and write something on the blackboard.

The chalk always left a residue on the board and on the erasers. It was quite a messy job to clean the erasers because the chalk dust would fly everywhere.

By jlmk — On Mar 15, 2011

I have always liked to collect rocks. I remember that when I was a child, I sometimes found rocks that I could draw on the driveway with. I remember thinking that these rocks were just like chalk. Now I'm thinking that they might actually have been deposits of natural chalk!

By roxytalks — On Mar 12, 2011

I would love to see the White Cliffs of Dover. I didn't know that they were actually formed out of chalk deposits. I always think of chalk as being such a soft material. It's hard to imagine cliffs being made out of it.

Is this natural chalk harder than what we would use for making chalk drawings, or is it the same? If you step on a piece of chalk, it breaks very easily. Although larger sidewalk chalk seems a bit more durable. Maybe that's how cliffs can be formed out of it -- the more there is, the stronger it becomes?

By claire24 — On Mar 10, 2011

I found this to be absolutely fascinating! I never knew that chalk was made from limestone and ancient sea creatures.

Why is blackboard chalk that is used in schools and on sporting fields made mostly from other mineral composites now, if natural chalk is so abundant? And is it true that sidewalk chalk made for children is not made from natural chalk anymore either? Is there something bad about natural chalk that would make it a poor choice for use with children?

I always like to use natural things when possible. So if there's nothing bad about natural chalk, I would like to find a place that sells chalk for kids that is made naturally.

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