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How To Identify A Crystal: Hardness

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There are several ways of how to identify a crystal or mineral. One of the most important 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!

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.

Friedrich Mohs created the Mohs Hardness Scale - image via Wiki
Friedrich Mohs – image via Wiki

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.

  1. OUTCOME ONE

    If Mineral A does scratch the surface of mineral B, then:
    Mineral A is harder than mineral B

  2. OUTCOME TWO

    But if mineral A does not make a scratch on mineral B, then:
    Mineral B is harder than mineral A.

  3. OUTCOME THREE

    Or, if mineral A makes a very small or difficult to determine scratch on mineral B.
    And if mineral B can only make a very small or difficult to determine scratch on mineral A. Then:
    Mineral A and mineral B are of relatively equal hardness.

  4. NOW YOU CAN BRING IN A MINERAL C.

    If mineral B can scratch mineral A, but cannot scratch mineral C, then:
    Mineral B is somewhere inbetween mineral A and mineral C.
    And so on!

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”.

Crystal hardness testing tools

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.

Crystal hardness Mohs Hardness Scale Diamond scratch test

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.

Testing crystal hardness scratches

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 Hardness Scale Testing Crystal Hardness Antique Kit
Mohs Hardness Scale Antique Testing Kit
Identifying crystals hardness 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).

Mohs Crystal Hardness Scale Number 1 Talc

2. Mohs Number Two: GYPSUM

Hardness level 2 is Gypsum. Gypsum is a soft sulfate mineral composed of calcium sulfate dihydrate.

You will know a variety of Gypsum as one of everyone’s favourite crystals – Selenite.

Crystals in the hardness 2 range include Baryte (2-3), Galena (2.5+) and Halite.

Selenite 220-320g 2

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.

Calcite, Clear Rough Pieces 60-80g 2

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).

Fluorite, Multicolour Rough Pieces 10-20g 5

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).

Green Apatite Rough Pieces XXS Pack of 10 >1g 1

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.

Sapphire, Blue Corundum Rough Piece 2-4cm 1 SP02 5

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.

Mohs Crystal Hardness Scale Number 10 Diamond

MOHS HARDNESS OF COMMON ITEMS:

Fingernail2 to 2.5
Copper sheet3
Nail3 to 6.5
Glass 4 to 7
Knife blade5 to 6.5
Steel file5 to 6.5
Streak plate6.5 to 7
Quartz7

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