E. Earth and Space Systems · Rocks, Minerals, and Geological Processes

Testing Minerals: Streak, Hardness and More

E2.3 A1.2 A1.4 A1.5About 45 minutes

Learning goals

I can use a streak test to find the colour of a mineral's powder.
I can use a scratch test and the Mohs hardness scale to compare how hard minerals are.
I can describe other properties, like cleavage, fracture and magnetism.
I can use the results of several tests to identify a mineral, and follow safety rules while testing.

Words to know

streak
The colour of a mineral's powder, made by rubbing the mineral on a rough white tile.

hardness
How well a mineral resists being scratched.

Mohs hardness scale
A scale from 1 (softest, talc) to 10 (hardest, diamond) for comparing mineral hardness.

cleavage
When a mineral breaks along flat, smooth surfaces, like mica peeling into sheets.

fracture
When a mineral breaks unevenly or with curved, rough surfaces instead of flat ones.

magnetic
Pulled toward a magnet. The mineral magnetite is magnetic.

Fool's gold

Long ago, some gold seekers found shiny, gold-coloured crystals and thought they were rich! But many of them had found pyrite, also called fool's gold.

How can you tell the difference? Not by colour: they are both gold-coloured and shiny. Geologists use tests. Today you will learn the tests that would have saved those gold seekers a lot of disappointment.

Remember from last lesson

A mineral is a natural, non-living solid that is the same all the way through. You already observed colour and lustre. Colour can fool you, because one mineral can come in many colours. That is why we need more tests.

Test 1: Streak

Rub the mineral across the back of a white tile that is not shiny (unglazed). It leaves a line of powder called a streak. The colour of the streak is the same every time for a mineral, even when the mineral itself comes in different colours.

Gold leaves a yellow streak.
Pyrite (fool's gold) leaves a greenish-black streak.
Hematite can look silver-grey or black, but its streak is red-brown!
Graphite, the mineral in pencils, leaves a grey-black streak. Every time you write with a pencil, you make a graphite streak.
Very hard minerals like quartz are harder than the tile. They leave no coloured streak (they may even scratch the tile).

A cluster of gold-coloured, cube-shaped pyrite crystals next to a white streak tile with a dark greenish-black streak. — Pyrite (fool's gold) is shiny and gold-coloured, but its streak on an unglazed tile is greenish-black. Real gold would leave a yellow streak. AI-generated image

Streaks on the back of a white tile. The streak colour stays the same for each mineral, so it is a better clue than the colour of the mineral itself. Gold and pyrite look alike but leave very different streaks.

Test 2: Hardness

Hardness means how well a mineral resists being scratched. A harder mineral can scratch a softer one, but a softer one cannot scratch a harder one.

In 1812, a scientist named Friedrich Mohs put ten minerals in order from softest to hardest. We call this the Mohs hardness scale.

Hardness	Mineral	Everyday test
1	talc	very soft; feels soapy
2	gypsum	your fingernail scratches it
about 2.5		fingernail
3	calcite	a copper coin scratches it
about 3.5		copper coin (an old penny)
4	fluorite	a steel nail scratches it easily
5	apatite	a steel nail just scratches it
about 5.5		steel nail or glass
6	feldspar	scratches glass a little
7	quartz	scratches glass easily
8	topaz
9	corundum	rubies and sapphires are this mineral
10	diamond	the hardest natural mineral

The Mohs hardness scale. Rows with bold words are everyday tools you can test with, placed where they fit on the scale.

The Mohs scale as a number line. Everyday tools fit between the minerals: a fingernail is about 2.5, a copper coin about 3.5 and a steel nail about 5.5. Each one scratches the minerals to its left.

Other clues

Cleavage: some minerals break along flat, smooth surfaces. Mica peels into thin sheets. Halite breaks into little cubes.
Fracture: other minerals break with uneven or curved surfaces. Quartz breaks with curved surfaces, like broken thick glass.
Magnetism: magnetite is pulled toward a magnet. Some black beach sand has bits of magnetite in it.
Fizzing: calcite fizzes in an acid, like vinegar. (You will try this in a later lesson.)

No single test is enough. Geologists use several tests together, then compare the results with a chart.

Worked example

Let's solve the fool's gold mystery

Two shiny, gold-coloured samples. Sample A leaves a yellow streak and is scratched by a copper coin. Sample B leaves a greenish-black streak and is not scratched by a steel nail. Which one is real gold?

Colour and lustre are the same, so they don't help.
Streak: gold's streak is yellow; pyrite's is greenish-black. Sample A matches gold. Sample B matches pyrite.
Hardness: gold is soft (a coin can scratch it). Pyrite is harder than a steel nail (about 6 to 6.5). Sample B is too hard to be gold.
Two tests agree, so we can be confident.

Sample A is gold. Sample B is pyrite (fool's gold).

Worked example

Let's narrow down a mystery mineral

A white mineral cannot be scratched by your fingernail. A steel nail does scratch it. It fizzes in vinegar. Use the Mohs table. What could it be?

Fingernail (about 2.5) does not scratch it, so its hardness is more than 2.5.
Steel nail (about 5.5) scratches it, so its hardness is less than 5.5.
On the scale, minerals between 2.5 and 5.5 are calcite (3), fluorite (4) and apatite (5).
It fizzes in vinegar. Of these, calcite is the one that fizzes.

It is most likely calcite, hardness 3.

Just for fun

What did the diamond say to the talc?

"You're such a softie!" (Talc is 1 on the Mohs scale, and diamond is 10.)

Safety for mineral tests

Do scratch tests on a table, with the sharp point of the nail pointing away from you and your hands. A grown-up should be beside you.
Skip the glass test at home. Glass can break and cut. Scientists use a special thick glass plate.
Hold the tile flat on the table when you streak. Tile edges can be sharp.
Never taste minerals. Wash your hands after testing, and wipe up the powder.
Keep small rocks and magnets away from little brothers and sisters. Small pieces can be a choking hazard, and swallowed magnets are very dangerous.

Hands-on activity

Mineral test station

You need: your rock collection, the back of a plain white tile (unglazed, from a hardware store; a grown-up can ask for a sample tile) or a piece of rough white paper if you have no tile, a steel nail, an old copper coin if you have one, a fridge magnet, a pencil, paper.

Warm up with graphite: scribble with a pencil on paper. That grey-black line is the streak of the mineral graphite!
Make a chart: Rock number, Streak colour, Fingernail scratch? (yes/no), Coin scratch? (yes/no), Nail scratch? (yes/no), Magnet? (yes/no).
Streak test: rub each rock firmly on the back of the tile. Record the colour of the powder (or "none").
Scratch tests: try to scratch each rock with your fingernail, then the coin, then the nail (with a grown-up). Wipe the spot: is there a real groove, or just a line of metal rubbed off the tool? A real scratch is a groove you can feel.
Magnet test: hold the magnet close. Does the rock pull toward it?
Use the Mohs table to put your rocks in order from softest to hardest.
Write one sentence about which test was the most helpful and why.

Note: many of your rocks are mixes of minerals, so different spots may give different results. Test the same spot each time, and record what you see. That is real science!

Your turn

What is a streak, and how do you make one?

Hematite looks silver-grey. What colour is its streak?

On the Mohs scale, which mineral is the softest? Which is the hardest?

Your fingernail scratches a mineral. What does that tell you about its hardness?

Which mineral peels into thin sheets?

How could you test if a black mineral is magnetite?

Why should you skip the glass scratch test at home?

Why do geologists use several tests instead of just one?

More practice: warm-up

What colour is gold's streak?

What is another name for pyrite?

What hardness number is quartz?

Which mineral is in pencils?

Can a softer mineral scratch a harder one?

More practice: on your own

Mineral X scratches mineral Y. Mineral Y scratches mineral Z. Put X, Y and Z in order from hardest to softest.

A mineral is not scratched by a copper coin, but a steel nail scratches it. Between which hardness numbers is it?

Quartz does not leave a coloured streak on the tile. Why?

What is the difference between cleavage and fracture?

A grey line appears on a rock after you rub a coin on it. Your friend says the coin scratched it. How can you check?

Challenge

A mineral has a metallic lustre, a red-brown streak, and a magnet does not pull it. What is it most likely? Explain.

Diamonds are used on the tips of some saws and drills. Use the Mohs scale to explain why.

Plan a fair test to put 4 minerals in order of hardness. What will you do, and what will you keep the same?

Check your understanding

Quiz

The streak of a mineral is...

On the Mohs scale, which is hardest?

Gold and pyrite are both shiny and gold-coloured. Which test tells them apart best?

Your fingernail cannot scratch a mineral, but a steel nail can. Its hardness is...

Mica peeling into thin, flat sheets is an example of...

Which is the safest way to do a scratch test at home?

For parents and teachers

Big idea (E2.3): Minerals are identified using several physical tests together: colour, lustre, streak, hardness (Mohs scale), cleavage/fracture, magnetism, and reaction to acid. Colour alone is unreliable.

How to help: Unglazed tile backs work well for streak tests; ask at a hardware or flooring store for a sample. Old copper pennies (Canada stopped making pennies in 2012, but many homes still have some; pennies dated before 1997 are mostly copper all the way through) make a good tester of about hardness 3.5. If you have no tile, the activity still works with the scratch and magnet tests. Many collected rocks are mixtures, so results may vary across a rock; praise careful recording rather than "right answers" (A1.2, A1.5).

Common difficulties:

Thinking the streak is the same as the mineral's colour. Hematite is the classic surprise.
Mistaking metal rubbed off a tool for a scratch. Wipe the spot and feel for a groove.
Reversing the scratch rule. The harder material does the scratching.

Safety (A1.4): Nail point away from the body, adult supervision, no glass test at home, careful with tile edges, wash hands, keep small rocks and magnets away from young children (swallowed magnets are a medical emergency).