The Mohs hardness scale is a relative measure of a mineral or gemstone’s resistance to being scratched. As a principal tool in gemology, the Mohs scale is important for identifying gemstones and preventing them from becoming damaged.
Where did the mineral hardness scale’s name come from?
German mineralogist and geologist, Freidrich Mohs, created the Mohs scale in 1812. At the time, most mineral classification was limited to chemical composition. While ancient scholar, Theophrastus, compared gem hardness first in 300 BC, Mohs created the first official classification list.
Mohs established 10 rankings: 1 as the lowest and 10 as the highest. It was common knowledge that the diamond was the hardest stone in the world, so he used it to represent 10. From there, he gathered 9 more readily available minerals with varied hardness levels to establish each ranking.
While the Mohs scale of hardness isn’t perfect, it’s a handy and essential tool for any gemologist or enthusiast.
In this guide, we’ll break down why the Mohs scale is important, how you can use it, and where each gemstone falls on it — from the softest minerals to the hardest.
You know hardness relates to scratching, but how do we define a scratch?
The Mohs hardness scale definition of “scratch” is a permanent, visible defect (or “dislocation”). If a stone’s toughness (covered next) is more elastic and goes back to its original shape and appearance after, then that isn’t a scratch in Mohs scale terms.
When Mineral A can scratch Mineral B, Mineral A will rank higher than Mineral B.
So, is an 8-level mineral twice as hard as a 4-level mineral? No, but that’s a common misconception! In actuality, the Mohs scale is a qualitative ordinal scale.
A quali-what now? A “qualitative” scale uses traits to describe something uncountable, rather than a numerical measurement. “Ordinal” scales are ordered by rank. Each level is simply higher or lower than another — you can’t precisely compare them with numbers.
In basic terms, the 1-10 on the scale could be replaced with A-J and essentially function the same.
The basic 1-10 Mohs hardness scale numbers don’t apply to all minerals, and there are some irregularities you may come across.
First, you may see in-between values like emerald’s hardness at 8.5.
Intermediate rankings are simply more specific rankings on the scale. In emerald’s case, it’s higher than the blue topaz hardness of 8 but lower than the rose quartz hardness of 7.
Another irregularity you might see is a hardness range.
For example, opal hardness is 5 to 6.5. Why? Opals have varied water content, making their composition inconsistent. Since chemical composition can affect the hardness, three different opals could be 5, 6, and 6.5, respectively.
Another factor that can affect gemstone’s Mohs hardness is weathering. If environmental forces (e.g. wind, rain, bacteria, etc.) wear down a mineral, it may become softer.
Lastly, you may see different hardness levels on the same crystal.
Kyanite crystals, for example, have a hardness of 5 on one axis and 7 on the perpendicular axis. The different hardness levels occur because one axis has weaker molecular bonds than the other.
Most people have heard diamonds are the strongest stone, but is that true? Diamond’s Mohs scale ranking is 10, making it the hardest mineral in the world.
However, hardest doesn’t mean unbreakable, because the Mohs scale is not a toughness scale.
Toughness (or tenacity) describes a mineral’s resistance to breaking or chipping.
Diamonds have brittle toughness, meaning they’re easily broken. There’s a common saying: A steel hammer can’t scratch a diamond, but it can break a diamond.
Let’s compare diamonds to jadeite jade. The hardness of jade is 6.5-7, so it’s easier to scratch than diamond. However, jade is tougher than diamond, so it’s easier to break a diamond than it is to break a jade.
Image: Vickers hardness tester
The Mohs scale is not the only gemstone hardness scale. Alternative scales include Rockwell, Vickers, Knoop, Shore, and Brinell. Each scale is most useful for specific types of material. The most popular are Rockwell and Vickers.
The Rockwell test uses a “differential-depth machine” to reveal indentation hardness. It measures how deep an indentation goes, based on leaving on a preliminary load, slightly increased load, then a major load. It’s the most accurate hardness test but requires advanced equipment.
The Vickers test is a microhardness or micro-indentation test, best for thin or small specimens. The Vickers process also measures indentation depth but much smaller indentations. However, the test requires smooth, polished specimens, advanced equipment, and calculations.
The Mohs test is most often used by gemologists and enthusiasts because it’s more accessible, it doesn’t require advanced equipment or calculations, and it’s the easiest to understand.
Back to the Mohs mineral scale, let’s see where every gemstone ranks!
The softest mineral on the Mohs scale is talc. From there, we have the standard reference for softest to hardest minerals:
Of course, there are more than ten types of minerals. Plus, non-gemstone items like metal, fingernails, pennies, and glass also have hardness measurements.
Let’s look at a complete hardness scale — minerals, gemstones, and other items included — from softest to hardest:
1-2 Graphite, Molybdenite, Pyrophyllite
2 Gypsum, Sylvite, Glauconite
2-2.5 Halite, Cinnabar, Chlorite, Fingernail
2.5-3 Silver, Gold, Copper, Chalcocite, Biotite
3 Calcite, Copper sheet
3-3.5 Witherite, Anhydrite
4-7 Glass, Nail
5-5.5 Titanite, Monazite
5-6 Uraninite, Turquoise, Ilmenite, Hornblende, Enstatite
5-6.5 Magnetite, Knife blade, Steel file
5-7 Streak plate
5.5-6 Sodalite, Nepheline, Chromite, Augite, Arsenopyrite
6-7 Zoisite, Epidote, Cassiterite
6.5-7.5 Sillimanite, Garnet
7.5 Zircon, Euclase
8 Topaz, Spinel
9 Corundum (Ruby, Sapphire)
Don’t see a certain gemstone on the list? Check for its mineral family!
For instance, jasper hardness is 6.5-7, the same as all chalcedony gems. Some families are more complex, like feldspar. Labradorite hardness, for example, is 6-6.5, unlike the standard 6 of most feldspars.
Let’s say you’re going on a trip and bringing some gemstone jewelry — maybe an amethyst necklace, diamond earrings, and fluorite ring. You put all the jewelry into one pouch and toss it in your suitcase. But oh no! Upon unpacking, you see your fluorite and amethyst gems are scratched up.
The above scenario is one example of why knowing the hardness of any precious or semi-precious gemstone is helpful for any jewelry lover. Had you known the hardness of amethyst is lower than diamond, and both are harder than fluorite, you could've prevented scratches by storing them separately.
Another area that gem hardness knowledge is crucial is lapidary work (gem cutting).
Lapidarists can’t just choose the most beautiful faceted cut — they have to plan their cut according to the gem’s hardness, toughness, and cleavage. Perfect cleavage, low toughness, and low hardness can make polishing and shaping a gem tricky, so proper lapidary technique and expertise are crucial.
The third important use for the Mohs hardness scale is identification.
Say you’re a rock or gem enthusiast hunting for gems (or “rockhounding”) in California, USA. You spot a green mineral you think could be jade — a valuable gem! Hoping to cash it in, you take it to a gemologist. Your hopes sink as the gemologist reveals it’s not jade but verdite, another green gemstone with a much lower value.
You would have spared yourself the rollercoaster of emotions if you’d brought along your own hardness testing tools!
Hardness testing is an easy first step for identification in the field. What tools are used to test a mineral’s hardness?
A standard Mohs hardness test kit has a divided container with 9 spaces, each holding rough crystals of the first 1-9 hardness minerals: talc, gypsum, calcite, fluorite, apatite, feldspar, quartz, topaz, and corundum. Diamonds are expensive, so they’re typically left out.
Another option is a set of sharp, metal hardness picks. These double-ended picks are often color-coded and labeled with each hardness ranking. Hardness picks are more compact and better at making precise, less conspicuous scratches. But, they’re pricier than standard testing kits.
Ready to become a Mohs hardness tester? Let’s begin!
Gather your testing kit, unknown mineral specimen, Mohs hardness chart, and a notepad to record results. If you’re indoors, conduct the test on a work table (not nice furniture) with a sturdy covering like rubber.
Step 1: Scratch the specimen with your fingernail.
Fingernails are 2-2.5 hardness, so if it leaves a scratch, you’ve narrowed the options down to minerals between 1-2.5. From there, scratch with gypsum, then talc.
No scratch? Move to step 2.
Step 2: Scratch the specimen with gypsum or a 2-level hardness pick.
Hold the unknown specimen with one hand as it rests on the table. You may want to keep the bottom (or any inconspicuous, flat area) of the specimen face-up to be scratched.
Take the testing mineral or hardness pick and put the sharp edge against the specimen. Keeping the specimen in place, firmly drag the testing mineral across the specimen. For safety, drag away from your body.
Wipe away any gemstone dust produced and check for a scratch. It should have a visible divot, so use a magnifying glass if needed.
Still no scratch? Go to step 3.
Step 3: Continue testing with each successive mineral or hardness pick.
Follow the instructions of step 2 but with your 3-level material, then 4-level material, and so on until you see a scratch.
When you do see a scratch, note which mineral made the scratch and its hardness. You can double-check by dragging the specimen against your mineral, which shouldn’t leave a scratch. From there, you can refer to your Mohs hardness chart to see what mineral your specimen could be. Still unclear? Conduct other identification tests to narrow it down.
Here are some more tips for ensuring a proper hardness test:
Always check for a divot in the scratch, not just a streak.
Try to choose at least medium-sized specimens to test.
If you suspect an unknown specimen may have undergone weathering, find a broken piece with the best luster to scratch.
Inclusions or surface blemishes may cause confusing results. If you think this may be the case, scratch a different area of the specimen.
Don’t forget you can always do a scratch test more than once!
With that, you’re all set to test away!
The benefits of knowing Mohs hardness rankings go beyond gemology experts or students. Any jewelry buyer can keep their gemstones shining by taking the gemstone hardness scale into account when storing jewels.
Once you start testing, who knows? Maybe you’ll discover gemology is your true calling! If you want to know more about gemstone testing, learn about the four essential testing tools!
SHOP OUR BEST GEMSTONES
Was this article helpful?9 people found this article helpful