Hardness is one of the basic properties Mineralogists (scientists that study minerals) look at to figure out what mineral they are working with.
They use a scale to quickly compare minerals. For instance, if they find a mineral and it has a hardness of 4, it can’t be Quartz, it’s too soft, Quartz has a hardness of 7.
The Moh’s Hardness Scale is the most common scale used by geologists and rockhounds in mineral identification. It was developed in 1812 by Friedrich Mohs, a German mineralogist.
The scale runs from 1 (the softest) to 10 (the hardest). Mohs picked 10 commonly known minerals as his “base.” He figured that most people who would be interested in using his scale could find those minerals and compare others to them.
| Hardness | Mineral Standard | |
| 1 | Talc |  |
| 2 | Gypsum |  |
| 3 | Calcite |  |
| 4 | Fluorite |  |
| 5 | Apatite |  |
| 6 | Feldspar |  |
| 7 | Quartz |  All kinds of Quartz, including agate, amethyst, and rose quartz. |
| 8 | Topaz |  |
| 9 | Corundum |  Corundum includes ruby (pictured) and saphire. |
| 10 | Diamond |  |
In comparison, a fingernail has a hardness of 2.5, and a steel nail has a hardness of 5.5. Most nails today are made of steel.
The two simplest tests looking at hardness are the fingernail test, and the steel nail test.
Can you scratch it with your finger nail? Then it’s very soft, less than 2.5. (Image on right is of fingernail scratching Gypsum)
Can you scratch it with a steel nail? Then it’s less than 5.5 on the hardness scale.
You then know the range of the hardness. Either it’s less than 2.5, between 2.5 and 5.5, or greater than 5.5. That should help you narrow your search for what kind of mineral you have.
Two naturally occurring minerals are harder than diamonds. Both were discovered since Mohs developed his scale. Wurtzite Boron Nitride, is 18% harder than diamond, and is sometimes found in volcanic rocks in very, very small amounts. Even harder is Lonsdaleite, it is 58% harder than diamond and found only in a few meteorites. They are the only natural minerals harder than diamond.
Below is a list of some common items and minerals and their hardness.
If a range of hardness is given, that means that the hardness depends on things that are sometimes mixed with the main substance.
2.5 fingernail – Does the mineral scratch with a fingernail, then its hardness is less than 2.5. Note though, that hardness of fingernails varies from person to person a bit, but almost always falls between 2.25 and 2.75.
2.5 - 3 Gold, and Silver
2.5 Copper Penny – Penny’s used the be thought of as having a hardness of 3, but recent testing found that at least some are really only 2.5. If you have some Calcite, you can test this – try to scratch the penny with the Calcite, and try to scratch the Calcite with a penny. That way you can tell if one is harder than the other – if they don’t scratch each other, then they’re both a hardness of 3.
4 Iron Nail – Most nails are steel, so you’d need to find one special. Try a good hardware store. If it scratches your mineral, but a copper penny won’t, that hardness is between 3 and 4.
5.5 Steel Nail – If a steel nail scratches your mineral, but an Iron nail does not, then your mineral’s hardness is around 5.
5.5 Glass – If your mineral will make a scratch on glass, then it has a hardness higher than 5.5.
6.5 Pyrite (Image at left is of pyrite scratching glass)
6.5 Hardened Steel File – If a hardened file won’t scratch your mineral, it’s very hard and has a hardness greater than 6.5. Usually above this level, you need to find a piece of mineral to scratch your mineral to determine the hardness. For instance, you need a Topaz to scratch minerals between 6.5 and 8 to see if the hardness is in that range.
