What Is the Mohs Hardness Scale? How to Scratch Test Minerals

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June 21

The Mohs Hardness Scale of Mineral Hardness helps identify minerals based on their relative resistance to scratching. It's a qualitative ordinal scale ascending from one to ten based on the minerals' hardness, AKA durability.

Friedrich Mohs, a German mineralogist, developed the scale in 1812. He arranged ten minerals in increasing order of hardness based on the ability of one mineral to scratch another. For example, Talc is the softest at 1, while diamond is the hardest at 10.

The Mohs Hardness Scale is a simple yet ingenious system for mineral identification. In this post, we'll explore its use and explain how to use it to differentiate minerals based on relative hardness.

How to Use the Mohs Hardness Scale

Use a scratch test to determine a mineral's durability using the Mohs Hardness Scale. Here are the general steps involved:

1. Purchase a Mohs Hardness Testing Kit: You'll need a collection of minerals of known hardness to conduct the test. The easiest way to accomplish the task is to purchase a Mohs Hardness Testing Kit. It should contain a specific mineral collection and a glass plate or porcelain tile.

2. Perform initial tests: First, try to scratch the unknown mineral with your fingernail (hardness 2.5). The mineral's hardness is less than 2.5 if your nail leaves a mark; and more than 2.5 if it does not. You can also perform this test with a copper penny (hardness 3) and a glass plate or steel blade (hardness 5.5).

Mohs Hardness Scale testing kit from Amazon.

Moh's Hardness Testing Kit from Amazon.

3. Conduct the scratch test: Start with a mineral of known hardness and try to scratch the unknown mineral with it. If that mineral leaves a scratch, the hardness of the unknown mineral is less than or equal to the known material. If the known mineral doesn't scratch the one of undetermined hardness, then the unknown material is higher than the known mineral.

4. Narrow down the range: Continue conducting the scratch tests with higher or lower hardness minerals, depending on your results, until you've narrowed the hardness to a specific range. For instance, if the unknown mineral scratches the glass plate (hardness 5.5), but not quartz (hardness 7), the mineral's hardness lies between 5.5 and 7.

It's essential to note that the Mohs scale only measures relative hardness, not absolute hardness. The steps between numbers do not represent equal spacing or relative differences. For example, diamond (10) is four times harder than corundum (9), which makes it much more difficult to scratch.

Examples of Moh's Hardness Scale Mineral Ratings 1-5

Here are some examples of different gems and minerals representing each number on the Mohs Hardness Scale:

Moh's Scale 1: Talc is the softest mineral on the scale, and all other minerals may scratch it. In turn, it can only scratch substances softer than itself, such as some types of plastic.

Moh's Scale 2: Gypsum is harder than talc, and all minerals above it on the scale may scratch it. Gypsum can scratch talc and substances softer than talc.

Moh's Scale 3: Calcite, represented by certain sea shells and pearls, is harder than gypsum and can scratch it and anything softer. Any mineral higher on the Mohs scale will leave a mark.

Moh's Scale 4: Fluorite can scratch calcite and anything softer than calcite. It can be scratched by any harder mineral, starting with apatite.

Moh's Scale 5: Apatite, a mineral group that includes turquoise, can scratch fluorite and anything softer. Any harder mineral may scratch it, starting with orthoclase feldspar.

Diagram of Mohs Hardness Scale with examples of minerals.

Moh's Hardness Scale Infographic.

Moh's Hardness Scale Examples 6-10

Moh's Scale 6: Orthoclase Feldspar, represented by gems like moonstone, can scratch apatite and minerals softer than apatite. Any harder mineral, such as quartz, may scratch it.

Moh's Scale 7: Quartz, which includes gems such as amethyst and citrine, can scratch orthoclase feldspar and anything softer than it. Quartz can be scratched by any harder mineral, beginning with topaz.

Moh's Scale 8: Topaz can scratch quartz and any minerals softer than quartz. It can be scratched by any harder mineral, starting with corundum.

Moh's Scale 9: Corundum, which includes gem varieties like ruby and sapphire, can scratch topaz and all minerals softer than topaz. Corundum can be scratched only by diamond, the hardest known mineral.

Moh's Scale 10: Diamond is at the top of Moh's Hardness Scale and can scratch all other minerals. No naturally occurring minerals are harder than a diamond. In that case, only another diamond or specific specialized, manufactured materials can scratch a diamond. Consequently, natural and lab-grown diamonds have the same degree of hardness.

Mohs Hardness Scale and Its Applications

While the intent of the Moh's hardness scale is primarily for mineralogy, it has found applications in many other fields. Its primary purposes are gem identification, metallurgy, and materials science. It's essential for design material selection because the hardness of a substance can determine its resistance to wear and tear.

Despite being over two centuries old, applying the Mohs Hardness Scale in material science and other fields proves its enduring relevance. It has limitations, as with any scientific tool, but its simplicity and practicality make it a fundamental mineralogical practice.

Understanding Moh's Hardness Scale enables us to appreciate the diversity of colored gems and minerals. It also allows us to gain insights into material properties essential in designing and selecting materials for various applications. Whether you're a budding geologist, a curious student, or a material design engineer, it's an essential resource.

Moh's Hardness Scale Faq.

Can Moh's scale be used for all materials?

The specific intent of Moh's Hardness Scale is mineral hardness testing. While it can indicate hardness for other materials, it is not absolute and doesn't accurately measure hardness outside the mineral realm.

What is the hardest mineral on Earth?

The hardest known mineral on Earth is diamond, which has a hardness of 10 on the Mohs scale. In other words, a diamond can scratch all other minerals and is highly resistant to scratching itself.

What is harder than a diamond?

Diamond is the hardest known natural mineral with a hardness of 10 on Moh's scale. However, synthetic materials like aggregated diamond nanorods may exhibit higher hardness levels.

What hardness level is glass on Moh's Scale?

On the Mohs hardness scale, glass typically has a hardness rating of about 5.5, meaning anything harder can scratch it such as steel or quartz.

How accurate is the Moh's Hardness Scale?

Moh's scale provides a relative, not absolute, measure of hardness. It's a simple and helpful tool, but other scales like the Vickers or Brinell hardness scales may provide more accurate and precise measurements.

Is 5.5 Mohs hard?

A hardness of 5.5 on Moh's scale is considered moderately hard and should scratch glass. However, a steel knife, which generally rates 5.5 hardness, may scratch a mineral of the same hardness rating.

How hard is 6.5 Mohs?

A hardness of 6.5 on the Mohs scale is relatively hard. It is the hardness level of many common minerals, such as feldspar, which can scratch glass and steel, but quartz may scratch it.

Is a 7 on the Mohs scale good?

A 7 on the Mohs scale signifies a hard mineral. Quartz, for example, is rated as seven and is hard enough to scratch glass and steel and resist wear and tear quite well.

What is a 10 on the Mohs scale?

A 10 on the Mohs scale signifies the highest level of hardness. The mineral diamond is rated 10, denoting that it is the hardest natural substance known to man.

About the Author

Dive deep into the glittering world of diamonds with Todd Gray, the CEO of Gray Matter Development, LLC. Todd has 35+ years of experience as a diamond buyer and trade consultant. He ghostwrites content for several online vendors and is an avid Freediver, currently exploring the Cenotes of Yucatan, Mexico. Dive into brilliance with Todd Gray!