What’s the Mohs Scale of Hardness?
In the early 1800s, Frederick Mohs developed a chart that compared the hardness of ten readily available minerals, beginning with talc (which scored the softest rating, 1) and working up to diamond (with the highest and hardest rating of 10). The chart is known as the Mohs Scale and is still used today, especially in the fine jewelry industry.
Rankings on the Mohs Scale show a comparison of hardness among the minerals. Naturally, the minerals with higher numbers will scratch minerals that rank below them on the scale. However, positions on this scale don’t indicate that minerals are equally close to each other in hardness. For instance, diamond (10) is four times harder than corundum (9), but corundum (9) is only twice as hard as topaz (8).
Original Mohs Scale Chart, from Soft to Hard
| Mineral | Mohs Rating |
|---|---|
| Talc | 1 |
| Gypsum | 2 |
| Calcite | 3 |
| Fluorite | 4 |
| Apatite | 5 |
| Orthoclase Feldspar | 6 |
| Quartz | 7 |
| Topaz | 8 |
| Corundum | 9 |
| Diamond | 10 |
If you’re anything like me, you might remember a Geology lab back in school where you used the Mohs scale to rank the hardness of random minerals you had in the classroom.
By taking one substance and attempting to scratch the surface of another material, you would be able to determine which of the two is harder. For example, let’s say you had a piece of glass (careful!) and a piece of talc. First you would try to scratch the talc with the piece of glass, and you would be successful. Then you would attempt to scratch the glass with the piece of talc, and you would be unsuccessful. Therefore, according to the Mohs Scale, the glass would be considered harder than talc. Eventually, you would be able to rank multiple substances in order of hardest to softest. Remember, hardness here is only judged based on the material’s scratch resistance, not its resistance to cracking or chipping, which is an important consideration when it comes to gemstones.
Today, geologists and gemologists have expanded the Mohs hardness rankings of minerals and other materials well beyond the original 10 that Frederick Mohs discovered. These rankings offer an important overview of the item’s durability as it relates to hardness.
Relative Hardness of Common Gemstones & Metals Chart
| Gemstone | Mohs Rating |
|---|---|
| Diamond | 10 |
| Sapphire | 9 |
| Ruby | 9 |
| Alexandrite | 8.5 |
| Topaz | 8 |
| Emerald | 7.5 to 8 |
| Aquamarine | 7.5 to 8 |
| Tourmaline | 7 to 7.5 |
| Amethyst | 7 |
| Smokey Quartz | 7 |
| Citrine | 7 |
| Garnet | 6.5 to 7.5 |
| Zircon | 6.5 to 7.5 |
| Peridot | 6.5 to 7 |
| Glass* | 6 to 7 |
| Bloodstone | 6.5 to 7 |
| Chrysoprase | 6.5 to 7 |
| Onyx | 6.5 to 7 |
| Moonstone | 6 to 6.5 |
| Labradorite | 6 to 6.5 |
| Turquoise | 5 to 6 |
| Lapis | 5 to 6 |
| Opal | 5.5 |
| Platinum | 4 to 4.5 |
| Coral | 3 to 4 |
| Gold | 2.5 to 3 |
| Pearl | 2.5 |
| Amber | 2 to 2.5 |
Why Should You Care About the Mohs Scale?
Knowing how gemstones rank on this scale helps you care for them in a way that prevents damage, but it won’t help you when it comes to gemstones chipping, because even diamonds chip and break. The Mohs Scale will help you make sure you don’t scratch your precious jewelry! Here are some tips:
- Jewelry Storage: Don’t store jewelry together with different Mohs ranks. For instance, sapphire scratches opal (and anything else rated below 9) and should not be stored in a location where the two can bump up against each other.
- Everyday Wear: At 2.5, pearls would suffer from the wear and tear of everyday use and should be treated with care.
- Engagement Rings: Diamonds, the hardest mineral, are an excellent choice for engagement rings and other jewelry that is worn continuously, and so are 9-rated ruby and sapphire.
Hardness isn’t the only factor to consider when you buy a gemstone, but it’s an essential ingredient to help you understand durability.
