If you’ve read any of my gemstone articles, you’ve probably heard me mention Mohs hardness about a dozen times. Now you get to learn what it’s all about…
What is Hardness?
Hardness is the ability of one mineral to scratch another.
(Don’t you love a simple definition?)
So, What is Mohs?
In 1812, geologist/mineralogist Friedrich Mohs established a qualitative scale of mineral hardness (see the chart on the right). In his honor, we call it the Mohs hardness scale.
Looking at the chart on the right, you see that talc has a Mohs hardness of 1. That means that all of the other minerals on the chart are harder than talc, and can scratch it. At the bottom of the chart, diamond is the hardest naturally-occurring mineral, with a Mohs hardness of 10.
One important thing to note about Mohs scale is that it is not a linear scale. For example, corundum is about twice as hard as topaz, and diamond is about four times as hard as corundum. It is a relative scale only.
How is Mohs Scale Used?
Mohs scale is an essential tool to field geologists, who use it to identify minerals. It is also used by gemologists to identify unknown gemstones. Each type of mineral or gemstone has a known Mohs hardness (or range of hardness values). In the field, mineral hardness is used in addition to properties such as luster, habit, and cleavage to determine the mineral type. Gemologists use density and an array of optical properties to identify gemstones.
If a gemologist has an unknown gemstone that can be scratched by quartz (7), but not by orthoclase (6), the hardness of the unknown sample is less than 7 and greater than 6. This information narrows down the possible type of gemstone.
Hardness vs Toughness
Hardness is not the same thing as the toughness of a material. For example, topaz has a hardness of 8, but it breaks somewhat easily. Jadeite, on the other hand, has a hardness around 6 or 7, but is very tough and doesn’t break easily. This is what makes jadeite an excellent material for carving.
Hardness vs Durability
The hardness of a gemstone is not the only the only factor that determines how durable a gemstone is. Durability is a determined by three factors: hardness, toughness, and stability. Jewelers prefer to use more durable stones to set in rings and bracelets, because these types of jewelry tend to get knocked around while being worn. Less durable stones are more suitable for setting in pendants and earrings.