What it shows
A harder material scratches a softer one. Using a fingernail, a copper coin, a steel nail and a piece of glass as reference points, any rock or mineral can be placed on the Mohs hardness scale from 1 to 10.
The science
Hardness is resistance to scratching, which depends on how strongly the atoms in a mineral are bonded. Mohs arranged ten minerals so that each scratches all those below it: talc 1, gypsum 2, calcite 3, fluorite 4, apatite 5, feldspar 6, quartz 7, topaz 8, corundum 9, diamond 10. A fingernail is about 2.5, a copper coin about 3.5, a steel nail about 5.5 and glass about 5.5 to 6, so these everyday objects bracket an unknown sample.
Materials
Cost: free. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- Rocks and minerals collected locally: chalk, brick, granite, marble chips, quartz pebbles, slate
- A copper coin or a thick copper wire
- A steel nail
- A piece of glass with taped edges
- A hand lens
- A notebook and labels
Safety
For this project: Tape the glass edges and let an adult hold it while testing. Wear safety glasses, because tiny chips can fly, and old clothes, because rock dust stains.
Never use mains electricity: power models with cells, batteries or a low-voltage adapter. Sharp and hot tools, flames and lab chemicals need an adult or your teacher beside you. Keep magnets, button cells and small parts away from young children, and wash your hands after making.
Step by step
8 steps. Do them in order, and test each part before you go on to the next. Write what you notice in a notebook as you go.
Wash and dry each rock sample and number it with a label.
Try to scratch sample 1 with your fingernail; if a mark is left, it is softer than 2.5.
If not, try the copper coin, then the steel nail, then try to scratch the glass with the rock itself.
Use the hand lens each time to make sure you see a scratch and not just powder rubbing off.
Record the hardest reference that scratched it and the softest it scratched, and write its hardness range.
Repeat for every sample and build a table from softest to hardest.
Compare the result for chalk, marble and quartz with the Mohs values for calcite and quartz.
Mount the samples in order on a card with their hardness ranges.
What you should see
A ranked display of local rocks. Chalk comes out softest, at about 1 to 2, marble around 3, brick and slate roughly 3 to 6 depending on the sample, and granite and quartz pebbles near or above glass at 6 to 7.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
01What is the Mohs scale?
A ranking of ten minerals from talc at 1 to diamond at 10 by what scratches what.
02Is the scale evenly spaced?
No; diamond is many times harder than corundum even though they are one step apart.
03Why check with a lens?
A soft sample can leave powder on a hard one that looks like a scratch.
04Which common mineral is hardness 7?
Quartz, found in sand and granite.
Take it further
Test a streak colour by rubbing each sample on an unglazed tile and add a streak column to the table.
Change one thing, measure what happens, and add a table or chart of your readings: that is what turns a model into your own investigation.
Free ebooks for this project
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