You may have already seen a "hardness" rating indicated on a gemstone's spec sheet, expressed as a number between 1 and 10. This figure refers to the Mohs scale, a simple but often misunderstood benchmark.
The origin of the scale
The Mohs scale was created in 1812 by the German mineralogist Friedrich Mohs. His goal was to classify minerals according to their resistance to scratching by comparing them to one another in a very concrete way: a mineral that scratches another is considered harder.
The scale ranges from 1 (talc, extremely soft) to 10 (diamond, the hardest known natural material).
What the Mohs scale actually measures
This is the most misunderstood point: the Mohs scale measures resistance to scratching, not resistance to impact.
A stone with a high hardness rating may be difficult to scratch, yet remain fragile if dropped or struck with a sharp blow. Despite its maximum hardness of 10, a diamond can crack under poorly angled impact. Conversely, a softer stone might resist a light shock without breaking.
In other words: Mohs hardness provides information on daily wear (friction, contact with other surfaces), not on the general robustness of a stone.
A few benchmarks for context
To give a concrete idea of the scale:
-
Talc: 1
- Human fingernail: about 2.5
- Pearl: 2.5 to 4
- Glass: about 5.5
- Steel (knife blade): about 6.5
- Quartz (amethyst, citrine): 7
- Topaz: 8
- Corundum (sapphire, ruby): 9
- Diamond: 10
These benchmarks help explain, for example, why a stone with a hardness of 7 like quartz holds up well against contact with sand or fabric, but can be scratched by concrete dust or a piece of steel jewelry stored poorly next to it.
Why it's useful to know this before buying jewelry
Knowing the hardness of a stone helps you take care of your jewelry intelligently:
- Storage: avoid storing stones of different hardness levels in direct contact with one another (a harder stone can scratch a softer one).
- Daily wear: a stone with high hardness (7 or above) withstands frequent use better than a soft stone like a pearl or opal, which are more delicate.
- Realistic expectations: high hardness doesn't mean "indestructible"; even a stone with a rating of 7 or 8 deserves to be handled with care.
In summary
The Mohs scale is a relative hardness benchmark useful for understanding how a stone resists daily wear, but it tells you nothing about its resistance to impact. It is one criterion among many to consider, rather than an absolute guarantee of durability.