The Mohs scale, developed in 1812 by Freidrich Mohs, a German mineralogist, measures the hardness of minerals. In this article, Benjamin Khordipour will reveal everything you need to know about the Gemstone Mohs Scale.
Important Note: It is not a linear scale – diamond is four times harder than corundum – but a ranking scale, comparing minerals to each other.
What is the Gemstone Mohs Scale?
The Gemstone Mohs Scale is a ranking system that measures the hardness of gemstones and minerals, indicating their resistance to scratching. It ranges from 1 (softest) to 10 (hardest), with the diamond at the top of the scale.
Here is a table of the Mohs Scale of Hardness for the most popular gemstones, along with their typical colors and hardness ratings. See below for the expanded list of gemstones and their hardness.
| Color | Mohs Hardness | |
|---|---|---|
| Diamond | Colorless, yellow, brown, blue, green, pink | 10 |
| Sapphire | Blue, pink, yellow, green, orange, purple | 9 |
| Ruby | Red | 9 |
| Emerald | Green | 7.5 – 8 |
| Pearl | White, cream, pink, silver | 2.5 – 4.5 |
| Aquamarine | Light blue, blue-green | 7.5 – 8 |
This list is just the most popular gemstones. See below for a comprehensive list of all the stones on the market.
Expanded List of Stones on the Mohs Scale
Here’s our list of the most common 50 stones and their placement on the Mohs Hardness Scale, organized by hardest to softest.
| Color | Mohs Hardness | |
|---|---|---|
| Diamond | Colorless, yellow, brown, blue, pink | 10 |
| Corundum (Sapphire & Ruby) | Blue, red, pink, yellow, purple | 9 |
| Chrysoberyl (including Alexandrite) | Yellow, green, brown, color-changing | 8.5 |
| Topaz | Blue, yellow, pink, clear | 8 |
| Spinel | Red, pink, blue, black | 8 |
| Beryl (Aquamarine, Emerald, Morganite) | Green, blue, yellow, pink, orange-pink | 7.5 – 8 |
| Zircon | Brown, red, yellow, green, blue | 7.5 |
| Tourmaline | Green, pink, red, blue, yellow, black | 7 – 7.5 |
| Quartz (Amethyst, Citrine) | Purple, yellow, clear, smoky | 7 |
| Iolite | Blue, violet | 7 – 7.5 |
| Peridot | Green | 6.5 – 7 |
| Tanzanite | Blue, purple | 6.5 – 7 |
| Garnet (Spessartite, Tsavorite) | Red, green, orange, yellow | 6.5 – 7.5 |
| Benitoite | Blue | 6.5 – 7 |
| Kunzite | Pink, violet | 6.5 – 7 |
| Lapis Lazuli | Blue | 5 – 6 |
| Turquoise | Blue, green | 5 – 6 |
| Opal | White, black, blue, green, red | 5.5 – 6.5 |
| Sodalite | Blue, white | 5.5 – 6 |
| Diopside | Green, brown, black | 5.5 – 6 |
| Bloodstone | Green with red spots | 6.5 – 7 |
| Moonstone | White, blue, peach, gray | 6 – 6.5 |
| Amazonite | Green, blue-green | 6 – 6.5 |
| Labradorite | Blue, green, yellow, orange | 6 – 6.5 |
| Hematite | Black, steel, gray, red | 5 – 6 |
| Apatite | Green, blue, yellow, pink | 5 |
| Fluorite | Purple, green, yellow, blue | 4 |
| Malachite | Green | 3.5 – 4 |
| Azurite | Blue | 3.5 – 4 |
| Rhodochrosite | Pink, red, brown | 3.5 – 4 |
| Coral | Red, pink, white | 3 – 4 |
| Chrysocolla | Blue, green | 2 – 4 |
| Jet | Black | 2.5 – 4 |
| Pearl | White, cream, pink, silver, gold, black | 2.5 – 4.5 |
| Amber | Yellow, orange, brown | 2 – 2.5 |
How to Determine a Gemstone Mohs Scale
You find the Mohs ranking of a mineral sample by taking two minerals – one from the scale and the mystery mineral – and trying to scratch one with the other. Hold the mystery mineral firmly on a flat surface, with a smooth, unscratched side facing up. Take the testing mineral and firmly scratch a pointed bit of it across the surface of the mineral you are testing. Brush away any dust and look for scratches.
Suppose A scratches B; A is harder. If A does not scratch B, then B is harder. If neither one is scratched, they are about the same. If A scratches B but doesn’t scratch C, then the hardness of A is between B and C.
Repeat the test to confirm your results. Continue through the sample kit until you find the point where one sample doesn’t scratch and the next one does. Your mystery sample’s hardness ranking is between those two. Harder minerals – the ones in the upper half of the scale – can require forceful pressure to test. Be careful to scratch away from yourself and your fingers.
The official ten reference materials are Talc, Gypsum, Calcite, Flourite, Apatite, Orthoclase feldspar, Quartz, Topaz, Corundum, and Diamond.
Why is the Mohs Scale Important?
Field geologists often carry a scratch kit containing samples of the ten reference minerals or a set of “hardness picks” scaled to the Mohs scale. The kits also include a list of rocks and minerals and where they fall on the scale. They use the scale to help identify minerals in the field.
The Mohs scale can also help you know how to care for the minerals in your jewelry. Minerals at different points in the scale require different care. You must be careful what you do with softer gemstones – opals are easy to break even by overheating them (doing the dishes, wearing them in a hot tub) or hitting them against something.
Estimates Using Common Objects
If you know the Mohs scale of some common objects you will likely have available, you can make a quick estimate even without a kit.
- Fingernail: 2-2.5
- Copper penny: 3
- Glass (window glass): 5.5
- Steel knife: 5-6
- Iron nail: 4-5
- Ceramic tile: 6-7
- Porcelain: 7
- Steel file: 6.5
- Sandpaper: 7-9 (depending on the abrasive used)
Limitations of the Mohs Scale
While the Mohs scale can tell how hard a stone is, that does not mean that it is durable. Hardness and durability (toughness) are different; a hammer at 5.5 on the scale can shatter a diamond if it hits at the right angle. Durability has to do with the gemstone’s crystalline structure. Several minerals have different crystalline structures—quartz is a good example. The various structures can have the same hardness but different durability.
Gemstones are often treated with oil or resins to enhance color and decrease flaws. These treatments can be used in sensitive areas of the stone. To know the best way to care for your gemstones, you need to know both scores – how hard and how durable – your kind of stone is and any treatments it has received.
How Hard Should the Gemstones in Jewelry Be?
Some gemstones are more appropriate for jewelry worn in more protected areas—necklaces, pendants, earrings—instead of areas likely to be exposed to wear and tear, like rings or bracelets. Gemstones worn in jewelry will likely be bumped into surfaces like door handles, countertops, or other jewelry. It should be at least a 7, which is the point at which they are unlikely to be scratched by common mineral dust.
The hardest mineral dust that is likely to be airborne is quartz – and quartz particles in dust are very common. Say you have a turquoise bracelet, and it gets dirty. You get a soft cloth to wipe it, but some dust is harder than the stone. Over time, it becomes dull and scratched up. It is essential to rinse gemstones, especially the softer ones, with warm water to remove the dust before wiping them off. Knowing the hardness of your stones will guide you in caring for them.
Where Common Gemstones Rate on the Mohs Scale
Let’s look at the hardnesses of gemstones commonly used in jewelry.
1 – 6 on the Mohs Scale
- 2 – 2.5. Amber is fossilized tree sap, and the softest regularly used gemstone. It is delicate and can be harmed by many common substances like gasoline, alcohol, and cleaning chemicals. It should never be cleaned with an ultrasonic cleaner.
- 2.5-4.5 – Pearls. Due to the way they are created – layer by layer inside a mollusk – they are easily broken and may dissolve in even mild acids. Cleopatra famously dissolved a pearl in vinegar and drank it to win a bet. Chlorine, perfume, and makeup can stain or damage pearls. You should clean them with a mild soap and distilled water (tap water contains chlorine).
- 5 -Multiple gemstones have a hardness between 5 and 6.
- Lapiz lazuli is a 5. Turquoise is 5-6. Turquoise is absorbent. If exposed to liquids like oils (say, worn while cooking), perfumes, or detergents, the color deteriorates, and you should clean these using only very mild soap and pure water.
- At 5-6.5, Jade is a soft but very tough gem. It is easy to scratch but easy for an artist to carve into gorgeous designs.
- The opal is 5.5- 6.5. Opals need moisture—up to 30% water—and if they dry out, they will crack and fade. They can also break or chip easily.
6 – 10 on the Mohs Scale
- 6 – Moonstone and Sunstone, both varieties of feldspar, are six on the Mohs scale. Avoid harsh chemicals, steam cleaning, or ultrasonic cleaning. Mild soap and non-chlorinated water are all that you should use to clean them.
- 7 – Many kinds of gemstones come in between 7 and 8 on the Mohs scale.
- Quartz has several possible crystalline structures and comes in many colors. These include amethyst, tiger’s eye, rose quartz, chalcedony, agate, carnelian, and citrine. Ultrasonic cleaners and steamers are safe with many kinds of quartz, though you should use mild detergent and warm water instead if you can.
- Garnets range from 7 to 7.5. They are sturdy but should still be cleaned with mild soap and water and kept from scratches from harder materials.
- Beryl ranges from 7.5 to 8, including aquamarines and the hard but fragile emerald. They are sensitive to household chemicals and hard knocks, and you should treat them gently.
- 8 – Topaz is one of the exemplars on the scale, harder than quarts and softer than corundum. It is not easily scratched, so you can wipe it off without worrying as much.
- 9 – Corundum includes rubies and sapphires. Washing and wiping corundum isn’t a problem as they are tough and durable. Avoid harsh chemicals, even with very hard gemstones.
- And, of course, diamonds are the perfect ten on the scale. (A few minerals are harder than the diamond; they are rare, often man-made, and used for industrial purposes.)
Final Thought on the Gemstone Mohs Scale
The Mohs scale is an excellent way to quickly identify the kind of mineral you are holding and give a rough idea of some of the care it should receive. It does not give the whole picture of that stone’s strengths and weaknesses, but it is a useful start.
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