Rocks and Minerals

Rocks and Minerals Facts

Rocks and Minerals [rŏks] [ănd] ['mĭn-ûr-'əlz]

Rocks are hard solids made of one or more minerals; minerals are a non-living, solid material with particles arranged in a crystal structure.

Rocks, rocks, and more rocks

Young boy is sitting on the rocks in the Mountains

Rocks are all around us. You can see rocks inside your house, in your yard, on your street, on a country road, everywhere you look. Statues, chalk, marble, pencil lead, sandpaper, glass, tombstones, bricks, the walls of your room, mountains, pebbles, soil, and volcanoes are all rocks!!

Humans have used the metals and minerals in rock since the beginning of civilization. Rocks are used to build homes, an aluminum baseball bat, a washing machine, video games, airplanes, cars, and jewelry! Rocks aren't always solid. Sand and mud are rocks. No matter where you are, you are always close to rocks and minerals. They are fascinating and exciting, so let's begin our investigation to learn more.

Let's start at the beginning …

Rock Formations Badlands National Park Rural South Dakota

We'll begin with a look at the structure of the earth because that is where all rocks come from. They have been on Earth for almost 4 billion years. Geologists record time with the Geologic Time Table.

Where do rocks come from?

Earth layers diagram

The Earth has 3 layers: the crust, the mantle and the core (which is subdivided into the outer and inner core). Each layer is unique. You can find the interesting details about the layers at The Structure of the Earth

How do we classify rocks?

There are 3 main types of rocks depending upon how they were formed in the different layers of the Earth. They are: Igneous, Sedimentary, and Metamorphic.

Rock Classifications

Type of RockHow is it formed?Where is it formed?Other Facts & Examples


Sedimentary rock is weathered into many pieces of rock and soil which then settles into layers. The layers are squeezed together until they harden into rock.

Found in locations where oceans, lakes, or other bodies of water exist or once existed.

Is layered, soft, and often contains fossils.

Examples: limestone, chalk, coal, sandstone, shale


Igneous rock is formed when melted rock cools and hardens.

Created by lava from volcanoes or magma that cools inside the Earth.

Can be shiny or glossy.

Examples: basalt, granite, pumice, quartz, obsidian


Metamorphic rock forms when igneous, sedimentary, or other metamorphic rock is changed by heat and pressure.

This type of rock is buried deep within the Earth where it is created from the heat and pressure found there.

Is hard, often contains crystals, and may have bands or layers.

Examples: marble, slate, gneiss, schist

Curious rocks in Barruecos

Perhaps this little “rock” song will help you remember the differences among sedimentary, igneous, and metamorphic rocks. Sing it to the tune of “Row, Row, Row Your Boat.”

Sedimentary rock has been formed in layers
Often found near water sources with fossils from 'decayers.'
Then there's igneous rock, here since Earth was born.
Molten lava cooled and hardened that's how it is formed.
These two types of rock can also be transformed
With pressure heat and chemicals, met - a - morphic they become.

Rock Poetry

A little boy stacking rocks at the beach

Rock On!
By Cherry Carl

Rocks! Rocks! They're not the same!
Each one has its claim to fame.
Rocks are grouped in three main types:
Some have layers … some have stripes.
What makes them different, do you know?
Let's take a look at rocks on show.

Lava in Hawaii

This rock star is “igneous.”
It's made when earth seems furious.
Liquid rock erupts and cools,
Forming solid glasslike pools.

Grey and pink gneiss metamorphic rock

“Metamorphic” means some changes.
Mother Nature rearranges.
Using heat and pressure, too,
She makes old rocks appear as new

wet gray pebbles on the seashore

“Sedimentary” means small pieces
That nature generally releases.
Using wind and water, too,
It makes new rocks for me and you.

Rocks! Rocks! You're not the same!
It's easy now to know your name.

Cherry Carl wrote this poem, especially for Science Trek. For additional poems on science, history, language, and holidays, visit her website at Carl's Corner.


A rock can begin as one type and can change many times. In fact, rocks are always changing. However, the changes happen so slowly that they are difficult to see. We have seen above that heat and pressure can change rocks, which then break down by weathering and move by erosion. It can take thousands of years for rocks to weather and erode. This process of change is called the rock cycle.

Colorful smooth stones on the shore

Weathering is the process that breaks rocks down into smaller pieces. Weathering can be caused by wind, rain, ice, running water, plant roots, chemicals, freezing, and thawing.

Hiker Passing Through the Water in a Gorge

Erosion is the movement of rock pieces from place to place. Erosion can be caused by wind, rain, running water, waves, gravity, and moving ice.

Fascinating Facts

Snapping Mt. Rushmore

The heads at Mt. Rushmore are carved out of an igneous rock called granite.

The heat from lightning striking beach sand can melt the sand to form a glassy rock called “fulgurite.”

Don't miss the amazing crystals at the Smithsonian Gem & Mineral Collection.

Melted rock is called magma when it is inside the earth, but called lava when it runs out onto the surface of the earth.

Different Kinds of Salts in Spoons

The mineral, salt, was so valuable in ancient times that it was traded ounce for ounce for gold.

A diamond is the hardest mineral.

Meteorites are rocks from space, and they can help scientists learn about the solar system.

What else do we know about rocks?

Quartz mineral sample

A rock is a material made of one or more minerals. Minerals are made from 92 elements that join together in many different ways. Some minerals are made of only one element, such as silver, but most are a combination of two or more elements. For example, the mineral quartz is made of the elements silicon and oxygen. The rock we know as granite contains the minerals quartz, feldspar, and mica. So, rocks are composed of one or more minerals, and minerals are composed of one or more elements.

Scientists have identified over 3,000 minerals. When the mineral particles are arranged in a repeating pattern, they are called a crystal.

Recognizing a mineral is not an easy job. There are several different properties of minerals and tests that are used to identify them. The properties include luster, hardness, color, streak, cleavage, crystal shape, and magnetism. You can view descriptions of these properties.

Want to learn more about the rock cycle?

The rock cycle is complex but you can learn a lot by revisiting the idea of it over and over. Check out these sites for more information:

  • The Mineralogical Society of America has a rockin' Mineralogy4Kids site.
  • has a great interactive page to study the rock cycle.

Discover more as you learn how to think like a geologist and mineralogist.

Top 10 Questions

May 2008

Thanks to Scott Hughes, Department of Geosciences, Idaho State University; and Virginia Gillerman, Idaho Geological Survey for the answers.

  1. How many different kinds of rocks are there?

    There are probably at least a couple thousand different kinds of rocks, but every one is unique. Rock is composed of a mixture of minerals, and if you've looked around outdoors you might even notice the granite in central Idaho looks a little different from one place to the next. It is estimated that 300 names help sort rocks into categories between sedimentary-, igneous-, and metamorphic-type rocks. (From Caleb in Mrs. Hooper's class at Gooding Elementary)

  2. What makes certain rocks valuable?

    Rocks are usually valuable either for their beauty and their decorative value, or else for a valuable element that they contain. Gold is the one that you might be thinking of, but many times we get valuable things from the earth that we may not appreciate, such as certain rocks that are used in jet engines and for lubricants and are important ingredients in many specialty steels. And so it's the use of something, whether it is sand or gravel, or gold, that makes it valuable. (From Tessa in Mrs. McCamish-Cameron's class at Cynthia Mann Elementary School)

  3. Why are rocks so hard?

    Rocks are hard because of the way their atoms are bound together. For example, you might have a rock that's made up of a mineral that has carbon. Carbon is one of those elements that, if it's bound a certain way, it makes a very soft mineral called graphite. But if it's bound a different way it can be so hard it becomes the hardest mineral known to humans, which is called diamond. Rocks are also hard as a result of the minerals they contain and how those minerals are bound together. For example, granite can be very hard because its minerals are intergrown and locking very tightly. Whereas a rock like sandstone that might not have its minerals packed together too tightly could be very soft. (From Rachel in Mrs. Whitesell's class at Gooding Elementary)

  4. Can diamonds melt?

    Yes, they could, if you could get it hot enough. While we are not sure offhand what the melting temperature of diamond is, any substance has a solid, a liquid, and a gaseous form. And that's essentially what melting is, going from the solid to the liquid. On the other hand, if a diamond were melted, it would be worthless after that because it would just be a blob of carbon liquid, and there's no way to make it back into a diamond, that's for sure. (From Taylor in Mrs. Gris's class at Horizon Elementary School)

  5. If diamonds are the strongest rock, what is the second strongest?

    Diamond is a mineral. And so the strong - the hardest - mineral is diamond. The next hardest one would be corundum. Well-known varieties of that stone would include the gem ruby, which is of course red, and emeralds, which are green. There is basic non-gem quality corundum, which is maybe not very common but not an uncommon constituent of typical metamorphic rocks like we have in northern Idaho. And it has a hardness of nine on what's called the Moh's (rhymes with toes) Scale of Hardness, which is the most common method used to rank gemstones and minerals according to hardness from 1-10. (From Kyle in Mrs. Gris's class at Horizon Elementary School)

  6. Why are some lava rocks red and others black?

    When lavas flow, like the lava flows one might see in Hawaii, they might be red because they're hot and glowing, but as the lava cools, the lava will become a black or an orange, or a red or brown, depending on the oxidation state. Iron, for instance, if you take your dad's hammer and you throw it out in the yard and leave it out in the rain and so forth, it gets rusty and it takes on a different color than the nice shiny steel when he brought it home from the hardware store. When lava cools, some of the gasses react with the iron in the lava to change the state of oxidation of that iron and so much of it comes out red. (From Jesse, a homeschooler in Filer)

  7. What is the biggest gem in the world?

    There are some gems that are quite large. The Hope Diamond is the most famous. It is a couple hundred karats cut, but of course it was probably several, maybe a couple of thousand, karats before it was cut and facetted into a cut gemstone. On the other hand, there are quartz crystals in museums that are almost gem quality, and some of those you could get one that was a foot long, and six or eight inches across. Some of the largest gemstone crystals come from Brazil, and you may have seen some of those in rock stores or in a museum. (From Montana in Mrs. Hammond's class at Camelot Elementary School in Lewiston)

  8. How do rocks form in the ocean?

    In the ocean rocks can form in two general ways. They can be what we call chemical precipitates, like a limestone, that might result from crystallization from dissolved carbonate parcels - particles that collect together, and then drop to the bottom of the ocean and form as layers of limestone. The other way, which is probably easier to see, is, for example, on a beach where you have a sandy beach, and over time if the ocean sort of retreats, that sand layer will get buried by other layers of sediment, and eventually over millions of years compressed into what could be a very hard rock. In the Boise foothills we have sandstone that was probably a lakefront beach at one time, and we can have finer grain sedimentary units as well. (From Sebastian in Mrs. Colburn's class at Endeavor Elementary School in Nampa)

  9. How do rocks get their shape?

    Rocks are formed by three different types of processes, and depending on the process, a rock will have properties making it softer in one direction, and harder in another direction. Or it might have some texture which we call foliated or lineated or layered, etc. As rocks break, they break along planes of weakness, so during the weathering process, for example "mechanical" weathering, the rocks are broken up and the shape depends on how strong they are in various directions. On the other hand, a rock may be washed over with water and eroded down so the surface of that rock might be changed just due to the erosion. It all depends on the property of the rock itself and how strong it is in one direction versus another. (From Joshua, a homeschooler in Boise)

  10. How do you find the age of a rock?

    There are two main ways geologists determine the age of a rock. The first way, of course, is with fossils. Fossils represent old animals or plants that evolved in certain directions, and paleontologists have studied that over the years and determined what the relative ages of those plants and animals are. More recently scientists have used radiometric age dating of rocks - that's a consequence of the decay of certain unstable isotopes of some elements. Rocks decay at a uniform consistent rate. By measuring certain isotopes within the rock and doing some mathematics you can calculate an age of the rock. So like we think the earth is about 4.5 billion years old, we can determine ages of rocks that are as young as 50,000 years on the Snake River Plain, for example. (From Alexandra, a homeschooler in Boise)