Why should you care about the Mohs scale? People ask me about the hardness of gemstones all the time. They know that some gemstones are harder than others, but have no idea how to determine whether amethyst or diamond is the stronger gemstone besides stepping on it or something. This is not recommended.
The jewelry industry uses the Mohs scale measurements to help you choose a gemstone that works with your lifestyle. You deserve to know what it will and won't tell you.
All About the Mohs Scale of Hardness
Way back in 1812, a German geologist by the name of Friedrich Mohs (big surprise, right?) created a “scratchability” hardness scale. The less “scratchable” a gem is, the higher its ranking.
The Mohs scale is about measuring gemstone hardness vs durability. Diamond tops the scale at #10. Talc rests at the bottom at #1. Nothing but another diamond will scratch a diamond. Just about everything scratches talc. A #8 topaz will scratch a #7 quartz, but not the other way around.
Basically, hardness—per the Mohs scale—is the ability to resist scratching. That’s it. Nothing more.
Mohs, an Ordinal Scale and What That Means
It's about to get technical. Just sayin. The Mohs scale is an ordinal scale. Ordinal means in the order it occurs. It does not mean that there is a identical distance between each item. This means that gems are listed by their hardness "order." The differences between levels, though, can vary widely. For example, the #10 diamond is four times harder than the #9 corundum (rubies and sapphires) but corundum is only two times harder than the #8 topaz.
Many people when looking at a Mohs scale don't realize that there are these uneven levels of hardness from one gemstone to another. Yup, there's nothing linear about it.
It's just that one gem type is harder than the next even if it's just a smidgen harder. Two gemstones one point away from each other on this scale can either be a tiny bit different, or a large amount different. There is nothing 'even' in the differences between one point and the next, for any of the gemstones on this scale.
Is Gemstone Durability and Gemstone Hardness the Same Thing?
Nope! Just because a particular gem is ranked higher on the Mohs scale, doesn’t mean it’s tougher or more durable than a different gemstone.
An example of this is the tsavorite gemstone. It is a softer gemstone in the 6.5 - 7 Mohs hardness range and scratches easily, however it is durable. You can beat that gemstone silly and it will scratch and get gouged, but it won't crack apart.
Then there's tanzanite which is 6.5 - 7 on the Mohs scale. But it differs a good bit from the tzavorite gemstone in the same 6.5 - 7 range. The tanzanite shares the same 'hardness' rating on the scale, but it is most definitely not durable, like the tzavorite. I accidentally dropped a loose oval tanzanite two feet onto a wooden table top fifteen years ago and it chipped badly.
I had to re-cut it as a pear shape. Ugg. But tanzanite is a gorgeous gemstone, so I put up with its perfect cleavage! (Cleavage defined in the next section.)
"Gems with perfect cleavage must be set carefully and worn carefully, as a sharp blow to the stone along a cleavage direction may easily split the gem." - Gem Society
When we consider toughness and durability we have to look at the crystalline structure of a gemstone. By this, I mean how the atoms line up and how strongly they bond with each other.
Cleavage in Gemstones and Why it Matters
Atoms often line up along one plane or another. We call these “cleavage planes,” and gemstones have a tendency to break or “cleave” along these flat, internal surfaces. Some gems break or cleave much more readily than others depending on this crystalline structure.
Even though the steel head of a hammer is only 5.5 on the Mohs scale, it can shatter a #10 diamond. The steel can’t scratch the diamond, but it can fracture the gem along its cleavage planes.
Interestingly, jadeite jade and nephrite jade are tougher than diamond which is why the mineral can be carved into such intricate and detailed sculptures. 5,000 years ago jade started being used for tool making. Jade was used in making some of the first ax heads. That's durability! Jade at 6-6.5 on the Mohs scale not as hard as a diamond which is 10 on the Mohs scale, yet it is more durable than a diamond.
Using the Mohs Scale to Choose Gemstones in Rings
Hardness and durability are important considerations in jewelry, especially with regards to rings. The Mohs scale will tell you the hardness, but not the durability of a gemstone. But it's the tool we have and so we use it.
Since rings tend to come into contact with hard surfaces—granite counter tops, metal door handles, other rings, e.g.—gemstones lower on the Mohs scale will scratch and get “cloudy” as they age. Examples of soft gemstones include: opals, garnets, and emeralds.
On the other hand, gems high on the scale, such as diamonds, rubies, and sapphires will retain their luster much longer and be more resistant to damage from hard surfaces.
How do We Know About Gemstone Durability?
The cleavage of a gemstone impacts how it will survive blows. This information is listed in many gem technical manuals. If your are curious about the wisdom of choosing to set a gemstone in a ring you'll wear daily, the easiest thing to do is talk to a Gemologist. Many jewelers can speak to the durability of the gemstone you are considering as well.
If you are considering various gemstones for your engagement ring you might want to read about forbidden wedding ring gemstones.
We jewelers appreciate that this scale was made. I wrote this so you'd know that the Mohs scale is not the be all end all of helpful things you should know about when choosing a gemstone. It is a helpful tool. Ask your jeweler if the gem you're considering is durable enough to survive your lifestyle and the way you plan to wear it.
Your Personal Jeweler,