Hardness is one of the three elements that are used to describe the durability of a gemstone (the other two being Toughness and Stability) but what do we mean by hardness?
Hardness is the ability of the surface of a gemstone to resist scratching and abrasion.
The Mohs Hardness Scale
We use the Mohs Hardness scale when talking about the hardness of gemstones, which was developed by German mineralogist Friedrich Mohs in 1812.
When doing hardness testing using the Mohs scale, we are testing a mineral’s resistance to being scratched relative to the 10 reference minerals, which are in the picture and table below that range from the softest (Talc) to the hardest (Diamond).
But there is one BIG problem with the Mohs hardness scale and that is the lack of information it provides as it doesn’t tell us how hard these gemstones actually are.
Because many people assume that the difference in hardness between Topaz and Corundum is the same as the difference between Corundum and Diamond but this isn’t the case and to explain why, we actually need to reference different hardness scales, with Vickers and Knoop being the two most usable for gemstones.
When looking at the results from Vickers hardness testing, it becomes obvious that the Mohs hardness scale is not linear:
| Gemstone | Mohs Hardness Score | Vickers Hardness |
|---|---|---|
| Diamond | 10 | 10000 |
| Corundum (Ruby & Sapphire) | 9 | 2085 |
| Topaz | 8 | 1648 |
| Quartz | 7 | 1103 – 1260 |
| Orthoclase | 6 | 714 |
| Apatite | 5 | 659 |
| Fluorite | 4 | 175 – 200 |
| Calcite | 3 | 105 – 136 |
| Gypsum | 2 | 60 |
| Talc | 1 | 47 |
When we see the Vickers hardness score, it gives us much more information about the differences in hardness between the minerals that were used for the Mohs hardness scale, with some of the more notable takeaways being:
This information is really useful but the problem is that outside of the results for the minerals on the Mohs scale, there isn’t any information about any other gemstones but why?
To put it simply, the Vickers (or Knoop, which is similar) hardness tests aren’t really suitable for gemstones as the test involves a small impact, rather than a scratch as with the Mohs test and as gemstones (especially crystalline) are generally brittle by nature, they have a tendancy to fracture when being tested, which isn’t ideal, which is why we still use the Mohs scale.
Vickers Hardness Scores source.
Gemstone Hardness Chart
A lot of this post has been focused on the 10 minerals that are used for the Mohs Hardness Scale but what about all the other gemstones?
Well I have compiled a list below of all the popular gemstones (in alphabetical order) and their hardnesses:
| Gemstone | Mohs Hardness Score |
|---|---|
| Agate | 7 |
| Alexandrite | 8½ |
| Almandine Garnet | 7 – 7½ |
| Amazonite | 6 – 6½ |
| Amber | 2½ |
| Amethyst | 7 |
| Andalusite | 7½ |
| Apatite | 5 |
| Aquamarine | 7½ |
| Aventurine Quartz | 7 |
| Azurite | 3½ – 4 |
| Beryl Varieties | 7½ – 8 |
| Benitoite | 6 – 6½ |
| Bixbite (Red Beryl) | 7½ – 8 |
| Citrine | 7 |
| Chalcedony | 6½ – 7 |
| Chrysoberyl | 8½ |
| Chrysoprase | 6½ – 7 |
| Cubic Zirconia | 8½ |
| Diamond | 10 |
| Diopside | 5 – 6 |
| Emerald | 7½ – 8 |
| Garnet Varieties | 6½ – 7½ |
| Fluorite | 4 |
| Grossular Garnet | 6½ – 7 |
| Hematite | 5½ – 6½ |
| Jadeite | 6½ – 7 |
| Jasper | 6½ – 7 |
| Jet | 2½ – 4 |
| Kunzite | 6½ – 7 |
| Gemstone | Mohs Hardness Score |
|---|---|
| Kyanite | 4 – 7 |
| Labradorite | 6 – 6½ |
| Lapis Lazuli | 5 – 6 |
| Malachite | 3½ – 4 |
| Moissanite (Syn*) | 9¼ |
| Moldavite | 5½ |
| Moonstone | 6 – 6½ |
| Nephrite | 6 – 6½ |
| Opal | 5½ – 6½ |
| Pearl | 2½ – 4½ |
| Peridot | 6½ – 7 |
| Pyrope Garnet | 7 – 7½ |
| Quartz | 7 |
| Rhodonite | 5½ – 6½ |
| Rhodocrosite | 4 |
| Rock Crystal | 7 |
| Rose Quartz | 7 |
| Ruby | 9 |
| Sapphire | 9 |
| Scapolite | 5½ – 6 |
| Smoky Quartz | 7 |
| Sodalite | 5½ – 6 |
| Spessartine Garnet | 7 – 7½ |
| Spinel | 8 |
| Tanzanite | 6½ – 7 |
| Titanite (Sphene) | 5 – 5½ |
| Topaz | 8 |
| Tourmaline | 7 – 7½ |
| Turquoise | 5 – 6 |
| Zoisite | 6 – 6½ |
How To Test The Hardness Of A Gemstone
To test the hardness of a gemstone, you need to buy a testing kit, with most modern kits using hardened metal alloys to replicate the hardness of the crystals (as opposed to a small bit of crystal being stuck to the end of a wooden stick, like they used to be) and they usually range from 2 – 9 on the hardness scale.
You then take your chosen mineral that you want to test and starting with the softest, run the point across the test piece to see if it scratches or not (best to do it on a small, inconspicuous spot) and then work your way up through the hardness levels until you find the one that does leave a scratch.
Once you find the hardness level that leaves a scratch, you then know that the hardness of the mineral that you are testing is slightly less or equal the level you have tested, for example:
If you test a stone and find that the number 6 scribe doesn’t scratch but the number 7 does, this means the hardness is above 6.
The big problem with hardness testing is that it is destructive as you are trying to damage the piece you are testing, which is why it is only usually done on ornamental objects that can’t be identified using other methods.
With cut gemstones, there are much more accurate and non-destructive ways to find out the identity of a gem.