The Mohs Hardness Scale is a set of ten reference minerals (numbered 1 through 10, where one is the softest and ten is the hardest) that are used to determine the relative hardness of minerals and other objects. In the jewelry industry, this scale is often used to provide guidance to consumers as to how resistant a stone in a given piece of jewelry will be. For example, a diamond (hardness of 10) will be capable of withstanding much more wear than a pearl (hardness of 2.5 - 4.5) in the same ring.
The hardness of a mineral is defined as its resistance to being scratched and is determined by observing whether a mineral’s surface is scratched by a substance of known hardness.
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Mineral
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Hardness
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Description
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Talc
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1
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Easily scratched by a fingernail; has a greasy feel
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Gypsum
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2
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Can be scratched by a fingernail
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Calcite
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3
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Very easily scratched with a knife and just scratched with a copper coin
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Fluorite
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4
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Very easily scratched with a knife but not as easily as calcite
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Apatite
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5
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Scratched with a knife with difficulty
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Orthoclase
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6
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Cannot be scratched with a knife, but scratches glass with difficulty
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Quartz
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7
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Scratches glass easily
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Topaz
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8
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Scratches glass very easily
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Corundum
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9
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Cuts glass
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Diamond
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10
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Used to cut glass
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Source: https://www.britannica.com/science/Mohs-hardness
Friedrich Mohs, a German mineralogist, developed the hardness scale in 1812. He selected ten minerals of distinctly different hardness that ranged from a very soft mineral (talc) to a very hard mineral (diamond). With the exception of diamond, the minerals are all relatively common, easy to obtain, and inexpensive.
The Mohs Hardness Scale has been used by geologists for over 200 years, with millions of students and scientists having used the Mohs Hardness Test to identify minerals. It is surprising that such a simple test has been in use for over two centuries.
The Mohs hardness test is conducted by placing a sharp point of one specimen on an unmarked surface of another specimen and attempting to produce a scratch. As part of the test, it is necessary to be certain that a scratch is actually made and not just a mark that will rub off. By doing successive tests with different reference minerals, you can closely estimate the Mohs hardness of your mineral specimen.
For example, if you have a mineral specimen of unknown hardness, and reference mineral #5 (apatite) can scratch it, then the hardness of your specimen is less than or equal to a Mohs hardness of 5. Similarly, if your specimen cannot be scratched by apatite, then its hardness is greater than or equal to a Mohs hardness of 5.
The Mohs Scale helps you to understand how resistant to scratching and breaking a gemstone is. This is very useful when you’re thinking of buying jewelry that will be worn daily, such as engagement rings, which often suffer a lot of wear and tear during their lifetime.
While most people take care not to hit their jewelry against a hard surface, scratching is a hazard encountered more frequently. Think of how many times every day you put your hands inside pockets, purses, glove compartments, and desk drawers, and what your ring may accidentally knock against while doing so.
Gemstone hardness contributes greatly to the degree to which a gem will show wear. For example, an opal with a hardness of 6 will be rapidly covered with fine scratches and lose its polish if worn everyday in a ring. On the other hand, a ruby with a hardness of 9 will remain bright and lustrous for years because it is harder than most of the surfaces that it will come against.
A 7 or higher on the Mohs scale usually indicates a gem is hard enough for normal jewelry use, although this is a guideline rather than a rule - pearls and opals are some of the most popular jewelry gems but are well below 7.
Diamonds (whether mined or lab grown) are the most durable choice for engagement rings, ranking a 10 in hardness.
Sapphires are the second-most popular gem for engagement rings and not by coincidence, as sapphires happen to be extremely durable, ranking 9 just below diamonds.
Rubies are formed from the same mineral as sapphires, called corundum, and therefore are also ranked 9 in hardness. Despite being rarer to see in engagement rings, they are highly suited for this purpose as well.
A disadvantage of the Mohs scale is that it is not linear; that is, each increment of one in the scale does not indicate a proportional increase in hardness. For instance, the progression from calcite to fluorite (from 3 to 4 on the Mohs scale) reflects an increase in hardness of approximately 25 percent while the progression from corundum to diamond (9 to 10 on the Mohs scale), reflects a hardness increase of more than 300 percent.
Additionally, if the mineral being tested is fine-grained or chalk-like, the test may only loosen grains without testing the mineral surfaces; therefore, certain textures may prevent a true hardness determination.