written by Diana Jarrett
In the jewelry trade we often refer to the Mohs Scale of mineral hardness for understanding a stone’s resistance to scratching. Gemologists rely on it as one of several signs for identifying an unknown gemstone. While you don’t want to randomly scratch an unknown stone to determine its Mohs hardness, there are a number of Mohs hardness test kits available to aid the gemologist today. Field geologists take their cue from a mineral’s relative hardness by scratching a known substance against a mystery rock.
The Mohs scale identifies 10 minerals, ranking them in order from the hardest, diamond at No. 10, down to the softest, talc, ranking No. 1.
Designers and manufacturers use the Mohs scale for determining what stones can safely be placed into certain jewelry items. Got kyanite? There’s a wide hardness range for this pretty material—anywhere from 4.5 to 7. So, specimens on the low end –at 4.5 hardness, are never suitable for rings or bracelets as they’re sure to scratch or chip under normal wear. Stud earrings or pendants are better suited for this and other soft stones.
At a store’s counter, retailers use a gemstone’s Mohs ranking as a valuable sales tool. “The sapphire –a corundum, is the hardest known substance next to a diamond!” We underscore how durable that gemstone is—and that it should remain beautiful for the wearer’s lifetime.
But, behind this commonly known name was a person. Carl Friedrich Christian Mohs, (1773-1839) a German geologist and mineralogist, originator of the Mohs scale enjoyed a prestigious career in the field of mineralogy long before he created the mineral hardness system in 1812.
By 1826, he was made full professor of mineralogy at the University of Vienna—while simultaneously being appointed curator of the Imperial Mineralogical Collection. His duties also included classifying minerals by their physical features rather than by their chemical composition, the more traditional method of the day. Mohs emphasis on physical traits was certainly at odds with the era’s prevailing chemical systematics. But practicality won out, and his arrangement, while not precise still reigns as the tried-and-true method of gem ID for our industry.
Helpful as the Mohs scale is to jewelers today, it wasn’t the first time such a procedure had been attempted. Actually, the process of comparing mineral hardness by observing which ones can scratch another is an ancient one, first mentioned by Theophrastus in his treatise On Stones, c. 300 BC. His work was the go-to source for other lapidaries until at least the Renaissance.
This ancient tome was later followed by Pliny the Elder’s Naturalis Historia, c. AD 77. Pliny investigated the problem of fraud and detecting false gems by using several tests, including a scratch test. He found that counterfeit gems (perhaps glass imitations) could be scratched by a steel file, whereas the real McCoy could not. He concluded that diamond sits at the top of the heap because, as he put it, “it will scratch all other minerals.”
Even with all the early precursors to the Mohs scale as we now have, the scientific study of gemology was a rather hit and miss endeavor until the latter part of the 19th century. One of the more legendary references to gemstone misidentification was the Black Prince’s Ruby—found in the Imperial State Crown of the United Kingdom. This massive 170 carat “ruby” dating to the mid-14th century, was later determined to be a red spinel. By the 18th century, enough gemstone separation data was available to give it its correct ID.
Portugal’s 1,680 carat Braganza diamond (also known as the Prince Regent Diamond) was discovered in Brazil in the late 18th century. It is said that Portugal’s King John VI had a hole drilled into the rough crystal, and wore it as a necklace on ceremonial occasions. Alas, it was later believed to be a white topaz—and unfortunately disappeared after the king’s death in 1826.
So much has evolved in the world of gemology since the early days of gemstone classification. Today we have a fail-safe system—albeit a sometimes complex one, for verifying a gemstone’s identity. With all the sophisticated advancements made—we can still send a word of gratitude to Mr. Mohs for organizing the hardness scale of gemstones we love.