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Today I have an awesome guest post for you today with a rock identification experiment. If you want a fun way to teach kids more about rocks and geology, read on!
Hi there! I still remember the first time I learned about this super useful experiment you can do when you are trying to figure out what kind of rock you have–the crazy thing is I was so old, but now I’ve taught this technique to kids of all ages, from high school to preschool!
One of our favorite activities lately is walking down to the river near our house, finding cool rocks, and figuring out what kind of rock we have or at least what minerals are in our rock. There are several little experiments you can do to help with this–today I’m going to show you how to do a scratch test with a steel nail.
My name is Carla Mae Jansen, and I recently finished writing A Dinosaur Made Me Sneeze, a picture book about the rock cycle (more on that below!). Anyway, since I’ve been working on this geology book we have been doing a ton of geology activities as part of our school time. Hunting for rocks by the river is always a favorite because you never know exactly what you will find!
Anyway, to do the scratch test with the nail, you just need one simple supply: a steel nail! (Parents, clearly this is an experiment you will want to supervise–do not turn your children loose with nails!)
Supplies for the Rock Identification Experiment:
- Steel Nail
- Rocks to test
How To to the Rock Identification Scratch Test:
1- Share this information with your kiddos before you go rock hunting, or at least before you start trying to identify what you found:
The scratch test works because all rocks and minerals have a different hardness–a harder rock will scratch a softer rock. There is a scale called Mohs Hardness Scale that ranks common items from 1-10, where 10 is the hardest and 1 is the softest. Here is a chart that shows ten common minerals and each of their hardness levels on Mohs Hardness Scale:
If one rock has a higher hardness than another rock, then the rock with the higher hardness will scratch the lower (or softer) rock.
Today I want to focus on 2 minerals that are very common, and show you how to tell them apart with the scratch test. They are feldspar and quartz. Afterwards, I’ll explain how you can use the scratch test to identify other rocks too!
2- The first step is to go find your rocks! We like to look near streams and beaches because the water always seems to bring something new and exciting! Quartz and feldspars are two extremely common minerals, so it’s particularly useful to be able to identify them! Plus, they make lovely rocks, so my kids often find them and want to know what they have.
3- The next step is to pull out your steel nail. First, scratch the rock. This is not a time to be soft! Use “moderate pressure,” or push pretty hard!
A steel nail has a hardness of about 6 and quartz is 7, so if you can scratch your rock, you do not have quartz. Feldspar and steel nails have a very similar hardness, so your nail might be able to scratch feldspars in some directions, but not others. You will also have to push very hard to scratch feldspar, but you should be able to do it.
Sometimes calcite or marble look like feldspar or quartz, but you can quickly eliminate them because a steel nail will easily scratch both!
You can also test calcite and marble with glass: glass has a hardness between 4 and 7, which means it will also scratch both calcite and marble but not either feldspar nor quartz. In fact, you can scratch a glass mason jar or plate with both feldspar and quartz!
4- Finally, you want to confirm your decision with other observations. Quartz grows as a beautiful crystal, but it usually breaks unevenly without a consistent fracture direction. Feldspars usually break along flat sides that often come together at about 90 degree angles. Colors don’t really help identify them because lots of other minerals can get mixed in with either quartz or feldspar and change the color. Sometimes you even get quartz and feldspar mixed together–this makes a beautiful “sparkly” rock, like this little piece of granite:
One other way to confirm your rock identification is if you have a pure quartz crystal. These are not hard to find or purchase, and they have a hardness of 7, which means they will definitely scratch feldspars (which have a hardness of 6)!
Try experimenting with a variety of rocks and minerals around you! How many have a hardness less than 6? Which are greater? Can you find quartz and feldspar in your area?
If you really enjoy the process or want to know the name of your rock, I’d encourage you to join a rock or mineral identification group–either online or in person (or both!). There are a lot of experts who enjoy the identification process and are happy to help beginners!
And if you want a fun starting point to chat about the rock cycle with your kids, you will want to check out my new picture book, A Dinosaur Made Me Sneeze!
This delightful rhyming story introduces rock cycle science (complete with vocabulary like “sedimentary,” “igneous,” “metamorphic,” and more!) with fantastic characters and a hilarious story! It showcases the changes that the Earth and rocks on the Earth make as years go by. You can learn more about it, see some of the illustrations, and more HERE!
I hope you have a “rockin’” time testing all your stones!!
Thanks again for letting me join you, and happy educating!
-Carla Mae Jansen
Carla Mae Jansen is an educator, author, and mom who lives in Virginia, USA. She loves going rock-hunting, eating chocolate, and exploring new places with her family. She has a master’s degree in teaching science, and is always looking for something new to learn! You can follow along with her publishing adventures at Turtle Trails Publishing.