Mohs Hardness Lesson Plan
Stonebridge Imports · Free Classroom Lesson Plan
Testing Mineral Hardness: The Mohs Scale
A hands-on inquiry lesson in which students predict, test and rank real mineral specimens by scratch hardness — then build a class Mohs scale from their own evidence.
Free to use, copy and photocopy for classroom use. No signup required.
Big Idea
Minerals can be identified by testing their physical properties. Hardness — a mineral's resistance to being scratched — is one of the most reliable and testable of those properties, because it does not change with the size, shape or colour of the sample. A tiny purple amethyst and a large clear quartz point are both quartz, and both scratch glass in exactly the same way.
Learning Goals
We are learning to…
- describe hardness as a physical property of a mineral;
- use a scratch test to compare two minerals and decide which is harder;
- order a set of mineral specimens from softest to hardest using evidence;
- explain how the Mohs scale lets scientists communicate hardness consistently;
- connect a mineral's hardness to how people use it in everyday life.
Success Criteria (co-construct with students)
- I can make a prediction before I test.
- I can carry out a fair scratch test the same way each time.
- I can record what I observe, not what I expected.
- I can put my specimens in order and justify the order with test results.
Curriculum Connections — Ontario
Grade 4 — Strand E: Earth and Space Systems, "Rocks, Minerals, and Geological Processes" (Ontario Science and Technology, 2022)
- E2.3 — "classify different rocks and minerals according to their composition and physical properties, using various tests and criteria." This lesson is a direct delivery of E2.3.
- E2.2 — "describe the physical properties of igneous, sedimentary, and metamorphic rocks."
- E2.4 — "describe everyday uses of rocks and minerals" (the closing application discussion).
- A1.2 / A1.4 / A1.5 — Strand A skills are taught inside Strand E: running an experimentation process, following health and safety procedures, and communicating findings using appropriate vocabulary.
Grade 12 — SES4U, Earth and Space Science (Ontario Grades 11–12 Science, 2008)
- E2.3 — "conduct a series of tests (e.g., hardness, streak, density) to identify and classify common minerals." Run the same lesson at senior level with streak, density and cleavage added.
- E3.1 — "identify the physical and chemical properties of selected minerals, and describe the tests used to determine these properties."
An honest note on the other grades. Rocks and minerals appear in exactly one place in the Ontario Grades 1–8 curriculum, and that is Grade 4. Grade 3 (Strand E, Soils in the Environment) builds the prior knowledge Grade 4 assumes. Ontario Grade 7 Earth and Space Systems is Heat in the Environment — it contains no geology; the "Grade 7 Earth's Crust" unit found in many lesson banks is Manitoba's. Curriculum references verified September 2026.
Materials
Per group — mineral specimens
- Selenite (gypsum) — Mohs 2
- Calcite — Mohs 3
- Fluorite — Mohs 4
- Obsidian or lapis lazuli — Mohs 5–5.5
- Labradorite (feldspar) or pyrite — Mohs 6–6.5
- Quartz (clear, rose or amethyst) — Mohs 7
Six specimens is the sweet spot: enough to make ordering non-trivial, few enough to finish in one period. Adding a spinel (8) or sapphire (9) lets students work the hard end of the scale too.
Per group — testing kit
- Fingernail (hardness ≈ 2.5)
- Copper coin or copper wire (≈ 3.5)
- Steel nail or paper clip (≈ 4.5)
- Glass plate or glass tile (≈ 5.5) — teacher-handled
- Unglazed white ceramic tile (streak plate)
- Hand lens
- Recording sheet and pencil
- Small tray or paper plate to contain fragments
⚠ Safety First
- Safety goggles for all scratch testing. Scratching produces small fragments and mineral powder.
- Obsidian is volcanic glass and can have genuinely sharp edges. Use tumbled or blunt pieces with younger students, or keep obsidian as a teacher-demonstration specimen only.
- Glass plate testing should be teacher-led or done at a supervised station — always place the glass flat on a table, never hold it while scratching.
- Scratch away from the body. Never test hardness against skin, desks or classroom equipment.
- Wash hands after handling specimens and before eating. Students should never taste or lick specimens.
- Do not crush or powder specimens. Scratch testing on an intact specimen is entirely safe.
Lesson Sequence
The disagreement
Hold up two specimens — say selenite and quartz — and ask: "Which of these is harder? How would you prove it to someone who disagreed with you?"
Collect students' proposed tests on the board. Expect "drop it", "hit it", "squeeze it". Draw out the crucial distinction: those tests measure how easily something breaks (toughness), which is a different property from how easily it gets scratched (hardness). Diamond is the hardest mineral on Earth, but strike it the wrong way and it will shatter.
Introduce the rule that makes hardness testable: if mineral A leaves a scratch on mineral B, then A is harder than B. Demonstrate once so every student sees what a real scratch looks like — and show the counter-case, where the softer mineral leaves a chalky streak that wipes away rather than a groove.
Part A — Predict (5 min)
Before touching any tools, each group arranges their six specimens in predicted order, softest to hardest, and records it. Ask them to note why — most will use colour, shine or weight, which is exactly the misconception the lesson will correct.
Part B — Test with tools (15 min)
Groups work through the kit in order of increasing hardness, recording ✓ or ✗:
- Can my fingernail scratch it? → hardness is less than 2.5
- Can a copper coin scratch it? → less than 3.5
- Can a steel nail scratch it? → less than 4.5
- Does it scratch the glass plate? → greater than 5.5
Circulate and press on technique: scratch firmly once in an inconspicuous spot, then rub the mark with a thumb. If it wipes away it was powder left behind, not a scratch. This single check is where most measurement error hides.
Part C — Mineral against mineral (10–15 min)
Now the specimens become the tools. Groups test which specimen scratches which, then build a final ranked order defensible from data alone.
Build the class Mohs scale
Draw a number line 1–10 on the board. Invite one group at a time to place a specimen where their evidence says it belongs, and to state that evidence aloud: "Our quartz scratched the glass, so it has to be above 5.5."
Reveal the standard Mohs scale and compare. Where groups disagree, treat it as data worth investigating rather than a wrong answer — genuine causes include a weathered surface, an impure specimen, or a composite rock rather than a single mineral.
Close with the application question: "Why is sandpaper made with garnet or corundum instead of calcite? Why are drill bits tipped with diamond?" Hardness is not trivia — it is why we choose one material over another.
Assessment
| Type | What to look for |
|---|---|
| Observation (as learning) | Is the group testing consistently? Do they rub the mark to check whether it is a scratch or powder? Do they revise a prediction when data contradicts it? |
| Recording sheet (for learning) | Completeness and honesty of the data table. Look for students who recorded a result that disagreed with their prediction — that is scientific behaviour worth naming and praising. |
| Exit ticket (of learning) | "A mineral scratches a steel nail but does not scratch glass. What is its approximate hardness, and how do you know?" |
Student Recording Sheet
Print this section (or the whole page) for students. Four steps: predict the order, test with tools, test mineral against mineral, then re-rank using evidence.
| Specimen name | Fingernail (2.5) | Copper coin (3.5) | Steel nail (4.5) | Scratches glass? (5.5) | Hardness is between… |
|---|---|---|---|---|---|
Teacher Answer Key
| Specimen | Mineral | Mohs | Fingernail | Copper | Steel nail | Scratches glass |
|---|---|---|---|---|---|---|
| Selenite | Gypsum | 2 | ✓ scratches it | ✓ | ✓ | ✗ no |
| Calcite | Calcium carbonate | 3 | ✗ | ✓ just barely | ✓ | ✗ no |
| Fluorite | Calcium fluoride | 4 | ✗ | ✗ | ✓ | ✗ no |
| Obsidian / Lapis | Volcanic glass / lazurite rock | 5–5.5 | ✗ | ✗ | ✗ marginal | ✗ or marginal |
| Labradorite / Pyrite | Feldspar / iron sulfide | 6–6.5 | ✗ | ✗ | ✗ | ✓ yes |
| Quartz | Silicon dioxide | 7 | ✗ | ✗ | ✗ | ✓ yes, easily |
Note the deliberate teaching moment at 5–5.5: obsidian and the glass plate are nearly the same hardness, so results are genuinely ambiguous. That is a real feature of the method, not a flaw in your specimens — use it to discuss measurement uncertainty.
The full Mohs scale
1 Talc · 2 Gypsum · 3 Calcite · 4 Fluorite · 5 Apatite · 6 Feldspar · 7 Quartz · 8 Topaz · 9 Corundum · 10 Diamond
Everyday reference points: fingernail ≈ 2.5 · copper coin ≈ 3.5 · iron nail ≈ 4.5 · glass ≈ 5.5 · steel file ≈ 6.5 · streak plate ≈ 7.
Corundum is worth naming: ruby and sapphire are both corundum — same mineral, hardness 9, with trace chromium making it red and trace iron and titanium making it blue. A memorable illustration that colour does not define a mineral; composition and structure do.
Exit ticket answer: Between roughly 4.5 and 5.5 — harder than the steel nail, softer than glass. Apatite, at Mohs 5, is the classic mineral in this range.
Differentiation & Extensions
- Scaffold: reduce to three widely-spaced specimens (selenite 2, fluorite 4, quartz 7) and pre-fill the prediction column so students focus purely on technique.
- Challenge: add a "mystery mineral" and have students determine its hardness range and propose an identity, justifying with at least two properties.
- Senior extension (SES4U): link hardness to atomic structure — why does diamond (pure carbon, rigid 3-D covalent network) sit at 10 while graphite (also pure carbon, but stacked sheets that slide) is a 1–2? Same element, opposite ends of the scale.
- Mathematics: the Mohs scale is ordinal, not proportional — a 10 is not "twice as hard" as a 5. In absolute terms diamond is roughly four times harder than corundum, which sits right beside it at 9.
Common misconceptions to watch for
- "Heavier means harder." Density and hardness are unrelated. Galena is very dense and quite soft; quartz is light and hard.
- "Shiny means harder." Lustre is its own property. Polished selenite is beautifully glossy and still scratches with a fingernail.
- "It broke, so it must be soft." Breaking is toughness or cleavage — fluorite has perfect cleavage in four directions and parts cleanly despite being harder than calcite.
- "A scratch on the glass means the glass is soft." Check by rubbing; mineral powder deposited on glass looks like a scratch until you wipe it.
Stonebridge Imports — Canadian wholesale rocks & minerals for schools, museums and STEM programs. Free to photocopy for classroom use. Companion plan: Modelling the Rock Cycle.