Testing Rock Hardness

Materials: ★★☆ Available in most school laboratories or specialist stores
Difficulty: ★☆☆ Can be easily done by most teenagers
Safety: ★☆☆ Minimal safety procedures required

Categories: Mining and Resources, Rocks

Alternative titles: Scratch Test: Rock Hardness

Summary

Students perform a scratch test to compare the hardness of different rocks. They discover that harder rocks can scratch softer ones and explore how rock hardness affects their practical uses.

Procedure

Begin by reviewing how rocks can be classified by observable features.
Show two rocks and ask students how they might test which is harder.
Demonstrate a scratch test by rubbing one rock against another; explain that a true scratch leaves a dent, while dust can be wiped away.
Allow students to predict which rocks will be hardest and softest.
Have students test at least two sets of rocks outdoors, recording results on a worksheet.
Guide students in ordering rocks from softest to hardest based on their tests.
Discuss how different rock hardness makes rocks suitable for uses such as carving, grinding, building, or tool-making.

Links

Fun Science Fridays episode 10: rock scratch test - Headwaters Science:

Identifying Minerals - Hardness Test - GeoscienceAustralia:

📄 Scratch test - Primary Connections: https://primaryconnections.org.au/teaching-sequences/year-3/dig-deep/lesson-5-scratch-test

Variations

Test classroom or local rocks (such as chalk, sandstone, quartzite) to compare hardness.
Use coins, nails, or glass to compare rock hardness against everyday materials.
Create a classroom hardness scale from 1 (softest) to 5 (hardest) based on tested rocks.
Use Mohr's scale and a test kit to test.

Safety Precautions

Conduct scratch tests outdoors to reduce exposure to dust.
Do not allow students to inhale rock dust.
Supervise use of hard or sharp rocks to avoid accidental cuts.
Wash hands after handling rocks.

Questions to Consider

What does it mean when one rock can scratch another? (It is harder.)
Why does dust not always mean a scratch? (Dust may come from the softer rock, not a true mark.)
How does mineral hardness affect rock hardness? (Rocks made of harder minerals resist scratching.)
Why might harder rocks be chosen for tools, while softer rocks are used for carving or building? (Harder rocks can cut or grind, softer rocks are easier to shape.)