Hardness

What is hardness??

In geology, hardness is a physical property that describes a mineral’s resistance to being scratched by another substance. It is a very useful tool in the identification of minerals. There is a lot of information out there on testing for hardness. Here I offer some observations and tricks I have learned over the years. Note you can use the Mindat Search page to help identify minerals.

The system most used is the Mohs hardness scale which lists 10 minerals from the softest (talc) to the hardest (diamond). See list to the right.

So for example, calcite is 3 so it will scratch gypsum and talc but will not scratch fluorite or anything higher.

Important note...The scale is not linear or even logarithmic. For example, fluorite is approximately 0.5 time harder than calcite but 10 times harder than gypsum (See the Mohs Wiki for more info).

1 - Talc 6 - Orthoclase

2 - Gypsum 7 - Quartz

3 - Calcite 8 - Topaz

4 - Fluorite 9 - Corundum

5 - Apatite 10 - Diamond

Here is the way I was taught to remember:

Those Girls Can Flirt And Other Queer Things Can Do.

How to test hardness.

It Is all about if the specimen will scratch a sample of known hardness or will be scratched by a sample of known hardness. For a "Known Hardness" it is best to use the minerals of the scale or a hardness tool set. I keep the following in a small box:

Calcite rhombohedral (hardness - 3)
Fluorite octahedron (hardness - 4)
Apatite (hardness - 5)
Orthoclase (hardness - 6)
Quartz crystal (hardness - 7)
Topaz (hardness - 8)

Note...a smooth surface is necessary for testing, so use a cleaved face or other smooth surface such as a crystal face as the test of the surface to be scratched. Use a point or edge of the mineral you are seeing if it will scratch this provides the most pressure per area.

Tip if needed, you can order the various minerals off eBay or Etsy. You can also find premade kits on Amazon for less than $20.

Be careful of using non-mineral for hardness. You will often see common objects included for hardness testing but be careful. In reality these objects can have hardness ranges. Here are some that are commonly listed:

Copper penny is often listed as 3 put I've seen a penny scratch calcite so it was over 3. Pennies have been reported to have a hardness between 2 and 3.5. If you use a penny use a pre-1982 non-wheat penny (these should have a hardness of 3). Also, being copper oxidizes / tarnishes make sure you scratch the penny and not just remove the tarnish.

Glass runs around 5.5-6.5.

Steel can range from 4 to 8.

If you have something you want to use that is not a mineral, test it against mineral. For example, I keep a utility knife in my kit. Steel can be between 4 and 8. The blade I currently have is not scratched by orthoclase but but quartz does so it would be considered a 6.5.

Some tips for checking hardness

Start with softer reference materials → protects specimen. if you are worried about damaging your specimen, start with the lower hardness reference material first so that your specimen will scratch it and not be scratched.
Test on bottom/back of specimen → preserves display surfaces. Use the bottom or back side of your specimen.
Did it really scratch it? If you think you have a scratch, rub it with a finger and see if it rubs off because sometime it is powder from the other. Also, run a fingernail over the scratch and see if you can feel it....this is where the knife blade or glass plate are useful.
Are multiple minerals? Often we are looking at minerals in matrix or mixed with other minerals so it can produce inaccurate results. For example limestone is mainly calcite (hardness of 3) but it can have quartz mixed in (hardness of 7)
Common substitutes:

Finger nail is approximately 2.5
Copper Penny (pre 1982) are approximately 2.5-3.5.
Glass is normally 5.5 but depending on the type can range from 5.5 to 6.5.
Knife blades, nails, files - I personally would not use these unless I tested them first as steel can have a large range depending on the type and treatment.

Something odd

Some minerals can have two hardness depending on the directions. For example Kyanite 5.5 parallel to the c-axis and 7.0 parallel to the a-axis. Also, Muscovite (2–2.5 vs. 4) and Hornblende (5–6).

If you have something you want to use that is not a mineral, test it against mineral. For example, I keep an utility knife in my kit. So it should be some place between 4 and 8. The feldspar does not scratch it but quartz does so it is between 6 and 7 so lets call it 6.5. Same for the dental picks.

The glass plate is not scratched by apatite (4) but is by orthoclase (6) so it is 5.5.

Note...a 5.5 does not mean it is halfway between 5 and 6. It just means it is between the higher and lower.

Additional Resources:

Determining the Hardness of a Mineral By Donald B Peck
Mohs scale - Wikipedia
Great short video by Nikki Barger (note this is a Facebook link so it may or may not work.

Side note......

If you are in the Mammoth Cave National Park area, you can hit me up and I normally have spares reference specimens and I'm happy to give out. Feel free to reach out to me at my contact.

Thank you

Thank you to the following people for reviewing this article:

Mark Gottlieb

Debbie Joplin

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