Hardness
Some minerals are harder than others. The hardness of a mineral is a good tool you can use to help identify minerals.
In 1812, a man named Fredrich Mohs invented a scale of hardness called Mohs Scale which is still used today. He selected ten standard minerals, and arranged them in order of increasing hardness. Talc is the softest and diamond is the hardest. Each mineral can scratch only those below it on the scale.
Look at the scale below - click on the pictures to find out about each mineral.
1. Talc 2. Gypsum 3. Calcite 4. Fluorite 5. Apatite 6. Orthoclase 7. Quartz 8. Topaz 9. Corundum 10. Diamond
You can easily test for hardness. Start with the softest standard mineral - talc. Scrape the talc across the mineral you want to identify. If it leaves a scratch, the mineral is softer than talc. If it doesn't, the mineral is harder than talc. Continue doing this with the harder standard minerals - gypsum, calcite and so on. If, for example, your mineral can be scratched by fluorite but not by calcite, it will have a hardness of about three and a half.
Some everyday objects can be used to give a rough guide of a mineral's hardness. Look at the example below. Pyrite and chalcopyrite have a metallic yellow colour and are sometimes called 'fool's gold'. You can tell them from each other and from real gold by doing the hardness test.
In the example above, gold has a hardness of just 2.5 to 3. It can be scratched with a copper coin. Chalcopyrite has a hardness of 3.5 to 4. It can be scratched with a steel penknife but not by a copper coin. Pyrite has a hardness of 6 to 6.5, too hard to be scratched with a steel penknife.
When testing for hardness, people sometimes mistake the powder of the softer mineral for a scratch on the harder one. When you test, make sure you brush away any powder before looking to see if there is a scratch.
It is very important to remember not to test for hardness on a good surface of your mineral - choose an area where any scratching will not show too much.
To find out about minerals you should Take a closer look, and then find out about some of their properties...
Colour
Light
Lustre
Streak
Crystal shape
Hardness
Cleavage and fracture
Heaviness
Other tests
What we have learnt
Return to the Detectives homepage, or finish by playing Mineral mastermind!
Copyright © 2006, Oxford University Museum of Natural History
