Introduction
In gemology, one important trait of gemstones is their durability. This matters because it relates to how beautiful a gemstone is. If a gem isn't pretty, people might not want it so that durability wouldn't be a big deal. But if a gemstone is beautiful, buyers or collectors usually want it to last a lifetime or even longer. Gemologists consider hardness part of durability, defining how well a material can resist scratching and wear. Understanding a gem's durability involves knowing its hardness compared to other materials around us.
What is the history of gemstone and mineral hardness?
For over 2,000 years, people have understood that some materials are harder than others. Around 77 A.D., Pliny the Elder wrote a book called Natural History, which classified minerals based on their physical properties. Before Pliny, about 300 years earlier, Theophrastus, a student of Aristotle who studied plants, wrote Peri Lithon, or About Rocks, which was an important book about gems and minerals.
In his writing, Theophrastus took a practical approach to the rocks and minerals he studied. Before him, many societies thought minerals had magical or mythical qualities. He focused on their physical characteristics and relative hardness, which are still relevant today. His detailed observations about the weight, shine, texture, hardness, and transparency of around fifty different materials were so influential that Friedrich Mohs used them as a key reference when creating the Mohs Hardness Scale 2,000 years later.
Who was Friedrich Mohs?
Friedrich Mohs was an Austrian geologist and mineral expert. He worked in various fields, including mining, helping private clients, and teaching. When he returned to work at a university, he improved how minerals are identified by looking closely at their physical features. His hardness scale is an important tool that people still use today to measure the hardness of gemstones.
What is the Mohs Scale of Hardness?
The Mohs Scale of Hardness measures how hard materials are, from 1 to 10, with 10 being the hardest. It uses 10 common minerals and gems to represent each number based on their hardness. Many materials can vary in hardness due to their type or impurities, and some fall between these numbers. Each of these 10 minerals can scratch those with a lower number and will be scratched by those with a higher number on the scale.
It's important to know that the hardness scale isn't linear. This means that a higher number isn't just a fixed amount harder than the one before it. For example, diamond, which is a 10 on the Mohs Scale, is four times harder than corundum (sapphire and ruby), which is a 9. Meanwhile, topaz, which is an 8 on the scale, is only half as hard as corundum above it.
What does the Mohs Hardness Scale tell us?
The hardness of a gemstone is important not only for those who identify it but also for anyone thinking about buying it. Some jewelry pieces will wear down more than others over time. For instance, necklaces and earrings don't get bumped or hit as much as rings. Some pieces, like engagement rings, are worn daily or for a lifetime. These rings should have more durable gems to last a long time.
The hardness of diamonds, rubies, and sapphires is a big reason why they are popular choices for engagement rings.
Hardness vs. Toughness
Many people think diamonds are the strongest stones. While diamonds rank 10 on the Mohs scale, which makes them the hardest mineral in the world, "hardest" doesn't mean "unbreakable." The Mohs scale measures hardness, not toughness.
Toughness refers to how well a mineral can resist breaking or chipping. Diamonds are considered brittle, which means they can easily break. There's a saying: a steel hammer can't scratch a diamond, but it can break one.
Let's compare diamonds to jadeite jade. Jade has a hardness of 6.5 to 7, so it scratches more easily than a diamond. However, jade is tougher than diamond, meaning it's harder to break jade than it is to break a diamond.
Reference minerals & chart
The Mohs scale of mineral hardness measures how well one mineral can scratch another. Minerals are pure solid substances found in nature, while rocks are made up of one or more minerals.
When the Mohs scale was created, diamond was known to be the hardest natural mineral, so it was set as a 10 at the top of the scale. To determine a material's hardness, we find the hardest material it can scratch or the softest material that can scratch it. For example, if a material can be scratched by apatite but not by fluorite, it would have a hardness between 4 and 5 on the Mohs scale.
When we talk about "scratching" on the Mohs scale, we mean making visible marks that don't bounce back. Sometimes, softer materials can leave tiny, invisible marks on harder materials. These tiny marks are permanent and can weaken the harder material, but they aren't counted as "scratches" on the Mohs scale.
Each of the ten hardness levels on the Mohs scale corresponds to a reference mineral, most of which are commonly found in rocks. The Mohs scale is a ranking system that orders minerals by hardness. For example, corundum (9) is twice as hard as topaz (8), while diamond (10) is four times as hard as corundum.
The table below shows these comparisons, along with the absolute hardness measured by a sclerometer and images of the reference minerals in the last column.
| Mohs Hardness | Mineral | Chemical Formula | Absolute Hardness |
|---|---|---|---|
| 1 | Talc | Mg₃Si₄O₁₀(OH)₂ | 1 |
| 2 | Gypsum | CaSO₄ · 2H₂O | 2 |
| 3 | Calcite | CaCO₃ | 14 |
| 4 | Fluorite | CaF₂ | 21 |
| 5 | Apatite | Ca₅(PO₄)₃(OH⁻, Cl⁻, F⁻) | 48 |
| 6 | Orthoclase Feldspar | KAlSi₃O₈ | 72 |
| 7 | Quartz | SiO₂ | 100 |
| 8 | Topaz | Al₂SiO₄(OH⁻, F⁻)₂ | 200 |
| 9 | Corundum | Al₂O₃ | 400 |
| 10 | Diamond | C | 1500 |
There are many more than ten types of minerals. In addition to gemstones, everyday items like metals, fingernails, pennies, and glass also have hardness levels.
Let's take a look at a complete hardness scale that includes minerals, gemstones, and other items, listed from softest to hardest:
| Hardness Range | Materials |
|---|---|
| 1 | Talc |
| 1 - 2 | Graphite, Molybdenite, Pyrophyllite |
| 1 - 3 | Bauxite |
| 1 - 5.5 | Limonite |
| 1.5 - 2.5 | Sulfur |
| 2 | Gypsum, Sylvite, Glauconite |
| 2 - 2.5 | Halite, Cinnabar, Chlorite, Fingernail |
| 2 - 3 | Muscovite |
| 2.5 - 2.75 | Galena |
| 2.5 - 3 | Silver, Gold, Copper, Chalcocite, Biotite |
| 2.5 - 3.5 | Barite |
| 3 | Calcite, Copper sheet |
| 3 - 3.25 | Bornite |
| 3 - 3.5 | Witherite, Anhydrite |
| 3 - 4 | Vanadinite |
| 3 - 5 | Serpentine |
| 3.5 - 4 | Sphalerite, Rhodochrosite, Pyrrhotite, Malachite, Dolomite, Cuprite, Chalcopyrite, Azurite |
| 3.5 - 4.5 | Siderite |
| 3.5 - 5 | Magnesite |
| 4 | Fluorite |
| 4 - 7 | Glass, Nail |
| 4.5 - 7 | Kyanite |
| 4.5 - 5.5 | Wollastonite |
| Hardness Range | Materials |
|---|---|
| 5 | Apatite |
| 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 |
| 5.5 - 6.5 | Hematite, Rhodonite, Diopside |
| 6 | Feldspar |
| 6 - 6.5 | Rutile, Pyrite, Prehnite, Nephrite Jade |
| 6 - 7 | Zoisite, Epidote, Cassiterite |
| 6 - 7.5 | Marcasite |
| 6.5 - 7 | Spodumene, Olivine (Peridot), Jadeite, Jade, Chalcedony |
| 6.5 - 7.5 | Sillimanite, Garnet |
| 7 | Quartz |
| 7 - 7.5 | Tourmaline, Staurolite, Cordierite (Iolite) |
| 7.5 | Zircon, Euclase |
| 7.5 - 8 | Beryl |
| 8 | Topaz, Spinel |
| 8.5 | Chrysoberyl |
| 9 | Corundum (Ruby, Sapphire) |
| 10 | Diamond |
Need help finding a specific gemstone on the list? Look for its mineral family! For example, jasper has a hardness of 6.5 to 7, which is the same as all chalcedony gems. Some families are more complicated, like feldspar. For instance, labradorite has a hardness of 6 to 6.5, while most feldspars are rated at 6.
Use of Mohs scale
Even though the Mohs scale isn't very precise, it is useful for field geologists. They use it to quickly identify minerals with scratch kits. You can often find the Mohs hardness of minerals on reference sheets.
The Mohs hardness scale is also helpful in milling. It helps determine which type of mill and grinding material will work best for reducing a product of known hardness.
In the electronics industry, manufacturers use the scale to test how strong flat panel display components are, like the cover glass for LCDs or the casing for OLEDs. They also use it to measure the hardness of touch screens in consumer electronics.
What are other factors to consider when selecting gemstones?
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Directional Hardness: Some gemstones have the same hardness in all directions, while others do not. This is called "directional hardness." The way a crystal grows depends on the atomic bonds between the elements it's made of. These bonds can be stronger in certain directions. For example, kyanite has very different hardness values depending on the direction: it's 4.5 in one direction and 7 in another.
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Brittleness: A gemstone like zircon has a hardness of 7.5, but it can get scratched easily if not handled carefully. Gem traders avoid storing faceted zircons next to each other because their sharp edges can damage and dull one another during transport.
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Toughness: Toughness refers to a gemstone's ability to resist breaking. Jadeite is a great example; it is so tough that ancient weapons were made from it, as it resists chipping or breaking. Although jadeite has a hardness of 6.5 to 7, it is exceptionally tough because it is made of many tiny, randomly arranged crystals. While the individual crystals may not be very hard, they are strong together.
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Rarity: If a gemstone is rare or hard to find, its desirability will depend on its durability. For example, the wing of a rare butterfly or the protective shell of a beetle may look beautiful, but they are not durable enough to be used as jewelry for long.
How to Test Mineral Hardness
Testing the hardness of minerals is a simple way to help identify them in the field. Here's how to do it and what tools you need.
A standard Mohs hardness test kit usually comes in a box with 9 sections, each containing rough crystals of minerals ranked from 1 to 9: talc, gypsum, calcite, fluorite, apatite, feldspar, quartz, topaz, and corundum. Diamonds are expensive, so they are usually not included.
Another option is to use a set of metal hardness picks. These picks are double-ended, color-coded, and labeled with each hardness level. They are more compact and can make finer scratches, but they cost more than standard test kits.
Hardness Test Step-by-Step:
Gather your testing kit, the mineral you want to test, a Mohs hardness chart, and a notepad to record your results. If you're indoors, use a work table (not your nice furniture) with a sturdy covering like rubber.
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Step 1: Scratch the specimen with your fingernail. Fingernails have a hardness of 2 to 2.5, so the mineral is between 1 and 2.5 if your nail leaves a scratch. If there's no scratch, move to step 2.
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Step 2: Scratch the specimen with gypsum or a hardness pick rated 2. Hold the mineral in one hand on the table, with a flat area facing up for scratching. Take gypsum or a hardness pick and press the sharp edge against the mineral. Drag it away from your body to avoid injury. Clean off dust and check for a visible scratch, using a magnifying glass if needed. If there's still no scratch, move to step 3.
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Step 3: Continue testing with each mineral or hardness pick in order. Follow the same instructions as step 2, but use your 3-level hardness material, then the 4-level, and so on until you see a scratch. When you find a scratch, note which mineral made it and its hardness. You can double-check by dragging your specimen against the mineral you just scratched with; it shouldn't leave a scratch. Then, refer to your Mohs hardness chart to see what your specimen might be. If you're still unsure, try other tests to help narrow it down.
Pro Tips for Testing:
Here are some tips to ensure a good hardness test:
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Always look for a visible divot, not just a streak, in the scratch.
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Use medium-sized specimens for better results.
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If the mineral looks weathered, find a broken piece with a shiny surface to test.
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Surface blemishes can affect results; try scratching a different area if you think this might be the case.
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Remember, you can always repeat the scratch test!
Now you're ready to start testing!
Conclusion
Understanding the Mohs Hardness Scale is important for anyone interested in gemstones. This scale helps us determine how durable a gemstone is and how well it can resist scratches and wear. While hardness is an important factor, it's not the only consideration. Other characteristics, such as brittleness, toughness, and rarity, also play a significant role in choosing the right gemstone for jewelry or collection.
Gemologists often rely on their observational skills rather than destructive testing methods to assess a gem's durability. This careful approach helps preserve the beauty and value of gemstones by using the Mohs Hardness Scale and considering other factors to make correct decisions when selecting gemstones that will last a lifetime, choosing gemstones for jewelry, or displaying them safely at home.
