Identifying a crystal by hardness. There are several ways of how to identify a crystal or mineral. One of these is by checking the material’s hardness. To do this, we use the ‘Mohs Hardness Scale‘. The who’s what now? Read on and be enlightened!
Table of Contents
Moh’s Hardness Scale
The Mohs scale was invented in 1812 by Friedrich Mohs, a German mineralogist.
He realized there needed to be a system for describing a crystal or mineral’s physical properties.
He was specifically interested in coming up with a way to describe whether the material was soft or hard, that was more specific.
That way, if some-one across the world found what they believed to be the same crystal or mineral, they could compare the hardness as one of the ways to help them properly identify it.
The system he came up with was very simple and very clever! He decided it would be useful to take 10 well known and relatively common minerals with differing hardness. Each of the 10 stones was slightly harder than its predecesor.
Thus, they would act as markers for 10 hardness levels that all minerals can be measured against. It is still the international standard to this day.
How To Test A Material’s Hardness
So, how did Mohs test to establish the hardness of each of the 10 minerals?
He used a scratch test to determine a mineral’s scratch resistence. Basically, he took the sharp end of one mineral (mineral A), and used it to try make a scratch on the other mineral (mineral B). There are three possible outcomes.
moh’s hardness scale scratch test: take mineral a, and try make a scratch on mineral b.
OUTCOME ONE
If mineral A does scratch the surface of mineral B, then:
Mineral A is harder than mineral B
OUTCOME TWO
But if mineral A does not make a scratch on mineral B, then:
Mineral B is harder than mineral A
OUTCOME THREE
If mineral A makes a very small, or difficult to determine, scratch on mineral B and visa versa, then:
Mineral A and B are of relatively equal hardness.
Now you bring in mineral C, and continue testing.
You can fairly easily determine where on the scale a particular mineral lies, using the base 10 minerals.
Knowing the material’s hardness level can help you identify what it is, or at least dramatically narrow your search, since there is a list of minerals that fall into each hardness level.
Hardness = Scratch Resistence
Mohs Hardness Scale very specifically looks at a mineral’s resistance to being scratched.
This is not the same as its toughness or strength.
A material might break, or crumble, or deform when you apply pressure. This is not the same as being scratched.
For the Mohs Scale you are only concerned with determining “scratchability”.
Problems With This Identification Method…
Arbitrary Choices
Not much in this world is completely infallible, and the Mohs Scale is, unfortunately, a little arbitrary. Remember these minerals were chosen and assigned by Mohs over 200 years ago, without much scientific data. It was based on what he considered to be fairly easily attainable.
They aren’t exactly spaced apart.
A more scientifically designed modern test would have each of the 10 minerals the same hardness difference apart, so the gap between 1 and 2, 2 and 3, 3 and 4 etc would be the same.
Also, the diamond, which is Mohs number 10, is not, as it turns out, the hardest mineral of them all.
According to a research report in 2009, it has been discovered that mineral wurtzite boron nitride may be harder than a diamond.
And lonsdaleite (itself made of carbon) is 58 percent stronger than diamond, setting a new record.
To be fair, these are both pretty rare minerals and you’re highly unlikely to ever come across them.
Method Variances
How the test is conducted is up to the individual.
How hard are you pressing? For how long? In what direction? How deep or big is a ‘scratch’ – does one need a microscope or just a magnifying glass to see it, or is it visible by eye? What if you unknowingly just aren’t putting enough pressure into the scratch?
These are undefined, arbitrary variances that dilute the test outcome from one person to the next.
Mineral Variances
Another problem is that a mineral does not have an exact, set hardness. Its hardness can be affected by where it formed, how it formed, what was (or was not) present, and so on.
Impuries can affect hardness. Weathering is a big influencer, and can definitely reduce a mineral’s scratch resistence. Some minerals can even have a greater or lesser hardness depending on the direction you move the scratch in, or what plane you scratch!
So, minerals generally have a variable hardness/scratch resistence in some degree.
Practicality
The last issue worth mentioning is that obviously not everyone has all 10 minerals readily available.
It’s also somewhat cumbersome to carry a “test kit” with you in the field.
Mohs does also have a HARDNESS OF COMMON ITEMS scale (see further down).
This means you can carry one item in your pack, of whose hardness is known, to test unknown samples while you are out collecting, to narrow down hardness level.
Some of these issues with identifying a crystal by its hardness can be offset by:
- Repeating the test a few times, and if possible,
- Using different pieces to test.
- Ideally try to collect a sample that is ‘fresh’/the least weathered you can find.
- And of course, practice and experience does help!
The Ten Minerals That Make Mohs Hardness Scale
1. Mohs Number One: TALC
The softest mineral to make hardness level 1 is Talc. Talc is a clay mineral, composed of hydrated magnesium silicate. It is (obviously) very easily scratched.
You will know talc it in its powdered form – as in baby powder, where it is combined with corn starch.
Crystals that are in the hardness 1 range include Sulphur and Limonite (1-5).
3. Mohs Number Three: CALCITE
At hardness level 3 is Calcite. Calcite is a carbonate mineral and the most stable polymorph of calcium carbonate.
Crystals other than Calcite that are in the hardness 3 range include Anhydrite (Angelite), Cuprite and Gold.
4. Mohs Number Four: FLUORITE
At hardness level 4 is Fluorite. Fluorite is the mineral form of calcium fluoride, belonging to the halide mineral group.
Crystals other than Fluorite in the hardness 4 range include Chalcopyrite (3.5-4), Malachite (3.5-4) and Azurite (3.5-4).
5. Mohs Number Five: APATITE
At hardness level 5 is Apatite. Apatite is actually a group of phosphate minerals, usually referring to hydroxyapatite, fluorapatite and chlorapatite.
Other crystals like Apatite in the hardness 5 range include Turquoise (5-6), Magnetite (5-6.5) and Sodalite (5-6).
6. Mohs Number Six: ORTHOCLASE
At hardness level 6 is Orthoclase. Orthoclase is a tectosilicate and a type of potassium feldspar, also known as K-feldspar.
Other crystals in the hardness 6 range include Pyrite, Prehnite and Nephrite Jade (all 6-6.5).
7. Mohs Number Seven: QUARTZ
At hardness level 7 is Quartz. Quartz is an igneous rock composed of silica. It is the second most abundant mineral in Earth’s crust after feldspar.
Other crystals with Quartz in the hardness range 7 include Tourmaline, Zircon and Beryl (all 7-7.5).
8. Mohs Number Eight: TOPAZ
At hardness level 8 is Topaz. Topaz is a fairly rare silicate mineral of aluminium and fluorine. It is a popular gem in jewellery.
Other crystals like Topaz in the hardness range 8 include Spinel and Chrysoberyl (8.5).
9. Mohs Number Nine: CORUNDRUM
At hardness level 9 is Corundrum. Corundum is a crystalline form of aluminium oxide typically containing traces of iron, titanium, vanadium and chromium.
Well known varieties of Corundrum you’d recognize are rubies and sapphires.
10. Mohs Number Ten: DIAMOND
Topping the chart at hardness level 10 is Diamond. Diamond is pure solid carbon, with its atoms arranged in a crystal structure.
Moh’s Hardness Of Common Items
| Fingernail | 2 to 2.5 |
| Copper sheet | 3 |
| Nail | 3 to 6.5 |
| Glass | 4 to 7 |
| Knife blade | 5 to 6.5 |
| Steel file | 5 to 6.5 |
| Streak plate | 6.5 to 7 |
| Quartz | 7 |
