Introduction: Rock Tests 101

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Rocks. They just sit around all day, so it's time to put them to the test. And if you thought tests were hard when you were younger, try when you're several million years old!

This is a set of rock tests to bring Earth Science to life for the budding young geologist to the expert explorer. A lot of textbooks talk about the scratch test for identifying rocks, but there is just so much more. From smelling and tasting to laser-ing, setting on fire, floating, and magnetizing, there's a lot to see in those wonderful minerals.

You may not have all the rocks in your classroom or home (yet!), but here is a growing list of things to do with them when you do.

  • What: Rock Tests 101
  • Why: Because the rocks aren't going to test themselves
  • Concepts: geology, electricity, buoyancy, optics, combustion, identification, magnetism
  • Time: varies by test
  • Price: cheap!

For getting rocks, there are many places online and probably around you. It is especially neat if you can figure out what comes from your country or state, and see if you can pick up your own!

If you want to do individual tests, that's great. If you want, you can combine them into a structured class activity (see final step) as well.

Test time!

Step 1: What Can You See?

The first part of examining rocks is looking for a story. Or a recipe. Every rock is composed of one or more minerals. Minerals are the inorganic ingredients that go into making up the Earth's crust.

To look for these ingredients, simply start with your eyes. Are there bands in the rock? Bubbles? Does it look like something you've seen before? Are there different parts? What colors do you see? Is it shiny? See if you can start to see what's going on descriptively before we get down to the testing stuff!

In these pictures, you can see a sedimentary rock with layers unevenly worn due to variations in hardness (1). In the next photo (2), you can see the boundary between where pumice becomes basalt in a lava flow. The next (3) shows petrified wood (all stone now), and then a geode containing crystals (4).

Now let's put these rocks to the test!

Step 2: Can You Read With Them?

If you have a set of rocks that you can see through, it might not be as easy as simple transparency. Many crystals have odd reflecting angles that bend light this way and that.

If you look through calcite, for instance, everything seems double. This is due to double refraction from birefringence, which is an amazing realm of optical science.

Ulexite, also known as "TV Rock" has an amazing ability to seemingly "project" light coming in from one side to the other like a television. It has parallel fibers that act like fiber optic cables to show an image from one side to the other.

You'll also find many interesting light-bending characteristics of quartz crystals and others. Just try to read with your rocks!

Step 3: What Happens With Special Forms of Light?

After taking a look in regular daylight, many rocks reveal interesting properties with other types of light. Going back to calcite, you can see the double refraction by sending a single laser beam through it.

Additionally, with ultraviolet light certain rocks take an amazing glow, often different than the color of the light being projected on it. Try fluorite, calcite, quartz, dolomite, and others to see these amazing properties. You can do this with a simple and cheap UV light bought from a party supply store. For a great rundown on UV rocks, check out this website. The UV rock collection photo comes from Dr. Hannes Grobe by way of this site.

Step 4: What Do They Smell or Taste Like?

You don't have to lick every rock that you see. But you may get some good minerals in your diet along the way. :)

Certain rocks appeal to our other senses as well, like smell and taste. Here you can see a block of sulfur with a strong yellow tint that gives off that eggy smell. Next to it as a block of rock salt also called "halite," just as it comes out of the mine before it gets ground up for the grocery store. Yum! Salt-sicles!

Step 5: Does It Float?

Most students would say that rocks sink. They'd be right for the most part, but then there's pumice. Pumice is an igneous rock with hundreds of tiny air bubbles in it that came from air infusion when it was in lava form. And it floats! And sometimes is used for self-scrubbing.

Here you can see pumice next to other igneous rocks such as basalt lava and obsidian. These are all chemically similar, just their forms and densities are different. They are often equated to being different parts of a shaken up bottle of Coke. The obsidian is like the dark liquid at the bottom, the pumice is the bubbles, and the basalt lava is the boundary in between.

Step 6: Is It Flammable?

Coal is one of the few organic rocks, composed mostly of hydrogen, oxygen, and carbon. With a propane blowtorch applied to the surface, you can see and smell bits of coal being burned. The energy that allows it to burn comes from good ol' ground up plants and animals from millions of years ago, and is still common today. Just check out your barbecue!

Other rocks can be at least melted at reasonable temperatures such as basalt. With a MAP gas blowtorch, you can even create your own lava! I'll get into that in another 'ible.

Use caution whenever working with hot materials, and make sure to keep observers back and with the proper safety protection.

Step 7: Does It Conduct Electricity?

Hoop up an LED with a battery and use a rock to complete the circuit! Only a handful of rocks are great conductors such as the metallic elements and graphite (you know, in your pencils). This a great way to separate true metals from metallic-appearing sulfides and oxides.

Here you can see galena conducting electricity quite well as well as naturally occurring copper. The pyrite, also known as "fool's gold," isn't doing seo well, even though it appears similarly metallic.

If you have a voltmeter, you can take it all a step further. Hooking up your wires, you can test the internal resistance of your rocks, giving you a better idea of their composition.

Step 8: Do Magnets Stick to It?

Grab a handful of magnets and see what they stick to!

They won't glom on to much, but they will to such things as iron ore. They'll also stick to anything that has ferromagnetic materials such as iron, nickel, and cobalt.

After you discover this, try seeing what magnets to see what metals around you they stick to. All of these things come from these rocks!

Step 9: What's Inside?

Some rocks look very different on the outside than on the other side. Grab a hammer and take a crack at them (chisel optional). To avoid flying rock flakes everywhere, put them inside a sock or piece of scrap cloth before you start hammering away.

In this case, I used a geode with quarts crystals inside that we gathered near Blythe, CA, but there are things such as thundereggs and even quartz crystals look amazing when hammered in to.

Step 10: Does It Bend?

We normally think of rocks as big sturdy things, but mica is a great wrench to throw in. It develops in sheets which are firm when all together, but can be cleaved apart into beautiful and bendy translucent sheets.

As you break it apart, you'll probably notice glitter-like flakes coming off of it. This is what they use to make some sidewalks glisten!

Step 11: How Does It React to Acid?

Check out the fizz!

With the addition of some low molarity hydrochloric acid (be careful!) or some vinegar, some rocks have quite the reaction. Here we can see the common limestone react with some HCl by bubbling violently. Next to it, you can even use plain vinegar on dolomite limestone, also known as "bubble rock" to make it fizz and even grow crystals.

This reaction is a neutralizing reaction whereby the limestone dissolves into carbon dioxide, water, and soluble calcium ions. When people talk about ancient statues "melting" in acid rain, this is what they're talking about.

Step 12: Can You Write With It?

Some rocks know how to write. Try limestone or some forms of tuff (volcanic ash) out, and you can leave your mark. You can ask students what the limestone reminds them of, because, well, limestone is used to make chalk!

Use some wood or a blackboard to write rock names, your names, or anything else.

Step 13: BONUS: Explore Fossils

Sometimes in the sedimentary, you see things that look not so rock-like. From clams to fish to whales to trilobites, when things fall down, sometimes their bodies become fossilized and preserved for a long time.

To extract these, you can use methods like applying acid to reveal them, cleaving rocks to uncover them, or take rubbings to document them.

Play around and use fossils to tell the story of where your rocks came from.

Step 14: Taking It Further

There are so many more tests to do. You can scratch test them, do hardness tests, look at them under microscopes, and more.

If you want to combine rock tests into a classroom activity, figure out what rocks you can get a hold of, and make a sorting chart. I made one for the rocks we had and titled the activity "Roberta's Rocks," whereby an earthquake hit Roberta the geologist's rock collection, and now they're all out of sorts. With these tests, she (and you) can sort them out again. Try making your own!

I'm excited to see what more you come up with, and we can always add on as a community. Feel free to add comments, questions, and corrections below!

Have fun, test those rocks, and as always, keep exploring!