04.01.2021 On the hardness of gemstones
When is a mineral a gemstone? When it is rare? When it is especially beautiful? In reality, most definitions of a gemstone include the mineral’s hardness.
In the past, stones that did not have a certain “gemstone hardness” were generally referred to as semi-precious stones. Today this term is rarely used commercially, as it can be misleading; generally one refers to jewels or jewellery stones.
To be a true gemstone, a jewel needs to have a Mohs hardness rating of at least 7.
MOHS SCALE OF MINERAL HARDNESS
Various different units of measurement, tests and scales have been developed over time for determining the hardness of materials. In mineralogy – and therefore also in connection with gemstones – the Mohs scale is generally used. This scale was developed by the German geologist, Friedrich Mohs, in the early 19th Century.
A hard material will scratch a softer material. Soft materials cannot, however, scratch harder materials. Based on this simple fact, Mohs developed his scale of scratching hardness.
Every position in the Mohs scale corresponds to a reference material, which is used to scratch another material. The scale is therefore not linear, which means, for example, that feldspar is not twice as hard as calcite.
|Scratches corundum, cannot be scratched by any other material
|Scratches window glass
|Can be scratched with a steel file
|Can be scratched with a knife
|Can easily be scratched with a knife
|Can be scratched with a copper coin
|Can be scratched with a fingernail
|Can be scraped with a fingernail
Minerals in gemstone hardness
Minerals with a Mohs hardness of 1 to 2 are considered soft, minerals from 3 to 5 are medium-hard, while those minerals with a hardness of 6 or more are hard. Minerals with a Mohs hardness of 7 or more are very hard and this is usually considered to be the minimum hardness to define a gemstone.
With a Mohs hardness od 10, diamond is the hardest material there is. Synthetic moissanite would theoretically be the second hardest gemstone, as it has a Mohs hardness of 9.5, but being synthetically produced it cannot be considered a gemstone.
The much sought-after ruby and sapphire are members of the corundum family and have a Mohs hardness of 9.
These are followed by the reference material topaz and also beryl, with a Mohs hardness of 7.5 to 8. Beryl includes emerald, aquamarine and other coloured varieties of the mineral. Spinel also has a hardness of 8, tourmaline lies between 7 and 7,5 and tanzanite is from 6.5 to 7.
Other hardness ratings
Alongside the Mohs scale, there are two other hardness scales that are sometimes used in connection with gemstones and minerals. These are the Rosiwal scale and the Vickers hardness test. August Rosiwal’s absolute scale measures the hardness of materials in terms of their abrasive resistance, while the Vickers test measures the material’s ability to resist plastic deformation from a standard force.