If you’ve ever heard the phrase “diamonds are the hardest material on earth” (and who hasn’t?), though you may not have known it, you were referring to the Mohs scale. The Mohs scale was created by a German mineralogist named Friedrich Mohs in 1822 to measure the relative hardness of minerals. He had been hired by a banker who had a very large collection of minerals, and he wanted Mohs to curate them. In order to properly curate the collection, Mohs had to sort the minerals, and at that time there was no acceptable method for categorizing minerals. Some scientists used color, others used geographic origin, and still others used various geometric characteristics.
Mohs noticed that some minerals scratched the surface of other minerals, and using that observtion he decided to explore their relative hardness. Though the minerology establishment at the time widely criticized Mohs for considering hardness as a meaningful characteristic, Mohs was ultimately validated. Today the Mohs scale is recognized as one of the most important measures of mineral categorization.
We’ve already mentioned that the diamond is at the top of the Mohs scale. Its measure of “absolute hardness” is 10. So what is at the bottom? Talc, with a measure of 1. We all know talcum powder, but some of the other elements in the Mohs scale — like fluorite and calcite — are less familiar to us in their mineral form. However, you probably do know minerals like quartz (7), and topaz (8).
Another good way to wrap your mind around Mohs hardness is to consider the Mohs rating of more every-day items:
- Maple or Oak hardwood floors are 1.3 – 1.4
- A fingernail has a hardness of 2
- The average knife blade has a hardness of 5
- The window panes in your house are 5.5
- Granite countertops are typically around a 7
- Tempered glass — like the average windshield — is usually a 7
- Bulletproof glass could be anywhere from 8 – 9
The Mohs scale is very important to us as we decide how to set various gemstones. Some gemstones are much softer than others, so how a gemstone will be worn must be considered. You’ve probably heard before that pearl rings are best saved for special occasions. Why? Because pearls are between 2.5 – 4.5 on the Mohs scale, so they scratch rather easily. Along with pearls, amber (2-2.5), coral (3-4) and malachite (3.5 – 4) all tend to last longest when worn in pendants or earrings because of their relative softness. Take more care in storing these softer gemstones by wrapping them in cloth or placing them in separate pouches.
At Omi Privé, we admire any gemstone that represents the best of its kind. If we find something rare and beautiful, we want you to see it. But for the most part, we work with gems that are a hardness of 7 or above, because these gems allow the most options for wearability. Diamonds may be the hardest on the scale at a 10, but rubies and sapphires are very durable at 9. Topaz, chrysoberyl, and spinel have a Mohs hardness of 8, and aquamarine and emerald are between 7.5 – 8. Quartz is a 7 and is the most common mineral on Earth. Because quartz is the most common mineral in dust, any gemstone softer than 7 can be scratched by common dust and, therefore, should be treated with more care than harder gems.
So the next time you’re looking at a gemstone, remember that it’s not just characterized by its color, its type, or its absolute beauty. Every gemstone also has a hardness rating on the Mohs scale, thanks to a fellow nearly 200 years ago who was faced with a sorting project.