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Mineral Identification: Hardness

Hardness is one of the basic properties used to identify minerals.

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.

It is a comparative scale, used to compare an unknown sample to things with a known hardness to see where it falls on the spectrum of hardness.

The Mohs 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.

The minerals of the scale, and their Mohs-designated hardness are:

In case you don’t have those handy, for 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.

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.

You can get a mineral hardness kit that will have a set of minerals from the scale (except diamond) that you can use to determine hardness.

Here are some materials with their hardness:

  • 2.5 fingernail – Does the mineral scratch with a fingernail, then its hardness is less than 2.5.
  • 2.5/3 Gold, Silver.
  • 2.5 Copper Penny – Pennies 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.
  • 4-4.5 Platinum
  • 4 Iron Nail – Most nails made today 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
  • 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.
  • 7 Quartz – This is the hardest commonly found mineral. As you can see, it is harder than glass and will scratch glass. Don’t use this as the only way to test that “diamond” ring.

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 diamonds.

Video Introduction to Hardness

Slides from the video: