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January 30, 2020
While the screen of a new iPhone 11 does a fine job resisting scratches from keys, coins and even razor blades, it would hardly stand up to a challenge from most of the gems in your jewelry box. The reason is that the glass of the iPhone 11 has a rating of 6 on the Mohs hardness scale, while your favorite peridot, topaz and diamond, to name a few, all rate higher.
More than 200 years ago, German geologist and mineralogist Friedrich Mohs developed a method to define the hardnesses of various minerals based on the relative ease or difficulty with which one could be scratched by another. Mohs’ method bucked the trend of classifying minerals by their chemical composition.
On the lowest end of the scale was talc with the rating of 1, and on the other end the spectrum was diamond with a rating of 10. Other minerals were given intermediate values based on their scratch performance.
It’s important to note that the Mohs ratings are relative, not linear, meaning that even though diamond is a 10 and talc is a 1, it does not mean that diamond is 10 times harder than talc. In fact, diamond is more than three times harder than the 9-rated corundum, which includes ruby and sapphire. And only a diamond can scratch a diamond.
When it comes to picking gemstone jewelry that is likely to be worn every day, such as an engagement ring, consumers are best served with a gemstone that resides on the higher end of the Mohs scale. When soft gems are used in jewelry, they need to be in settings that will protect them, and worn with extreme care.
It’s also important for jewelry lovers to remember to protect softer gems from scratching, especially when they are sharing a jewelry box with harder items, such as diamonds. The gems most susceptible to scratching are pearls, amber, malachite, coral, opal and turquoise — all of which rate 6 or lower on the Mohs scale.
Experts recommend storing gemstones apart from each other, such as utilizing separate compartments of a jewelry box, or separate boxes altogether. Gems can be further protected from the edges of metal jewelry by wrapping them in tissue.
Here’s a sampling of where your favorite gemstones rate on the Mohs scale:
9 Corundum, including sapphire and ruby
8 Topaz, spinel
7 Quartz, citrine, amethyst
6.5-7 Peridot, tanzanite
5.5-6 Opal, turquoise
3.5-4 Malachite, coral
Compare the hardness of the gems, above, to these common items…
8.5 Masonry drill bit
6.5 Steel nail
6 Screen of iPhone 11
5.5 Razor blade
5 Window glass
3.5 Copper penny
2.5-3 Pure gold or silver
During his illustrious career, Mohs earned professorships in Graz (1812), Freiberg (1818) and Vienna (1826). The memorial plaque shown above is dedicated to Mohs and his accomplishments in the Austrian capital. In 1839, Mohs passed away while on holiday in Italy at the age of 66.
Credits: Portrait of Friedrich Mohs by Josef Kriehuber [Public domain]. Marquise Diamond Ring photo by Chip Clark and digitally enhanced by SquareMoose/Smithsonian. Mohs scale graphic by Eurico Zimbres [CC BY-SA]. Memorial plaque photo by Doris Antony, Berlin [CC BY-SA].