The Mohs Scale

The Mohs Scale

Written by on 21 September 2013

The Mohs scale is a tool used to identify the hardness of gemstones, invented by the German mineralogist Friedrich Mohs in 1812. As a mineral curator, after having completed his studies, his role was to categorise minerals, comparing them and examining them for common properties. He gathered a selection of minerals, from Talc to Diamond which he knew varied in hardness, and began to measure their hardness using a scratch test. He developed a scale of hardness from one (softest) to ten (hardest), which is not linear and not strictly exponential but with varying degrees of increase in hardness between numbers. So the increase in hardness from numbers 3 to 4 is about 25%, whereas the increase in hardness from 9 to 10 is more than 300%.

Although Friedrich was the first to develop this official scale, he was by no means the first to think of it! It was mentioned by Theophrastus c. 300 BC, followed by Pliny the Elder c. 77 AD!
In this case, 'hardness' is the resistance of a material to be scratched. So when two materials are tested, one is sharpened to a point and scratched onto the other's surface. This process determines the hardness of the minerals relative to each other:
1. If Specimen A can scratch Specimen B, then Specimen A is harder than Specimen B.

2. If Specimen A does not scratch Specimen B, then Specimen B is harder than Specimen A.

3. If the two specimens are equal in hardness then they will be relatively ineffective at scratching one another. Small scratches might be produced, or it might be difficult to determine if a scratch was produced.

4. If Specimen A can be scratched by Specimen B but it can not be scratched by Specimen C, then the hardness of Specimen A is between the hardness of Specimen B and Specimen C.

Geologists will often use this scale to identify a mineral. In the field, they would conduct the scratch test on a mineral with a stainless steel knife, Mohs scale 5 to 6.5, and therefore conclude that the mineral is either harder or softer than stainless steel. Alternatively, wherever you are, you are never very far away from quartz which also proves to be a handy hardness comparison tool!

These days it is common practice to use a set of hardness picks made from metals and alloys equal to various numbers on the Mohs scale to carry out the same tests on minerals with more accurate results.

A specific mineral will usually result in a consistent hardness, but some can be tricky! Some minerals can vary in composition, like Garnet, which can have several different compositions, will vary in hardness from 6.5 to 8. Some minerals have a different hardness at different angles due to their crystal formation. For example Kyanite, which is formed in blade like crystals, has a Mohs scale of 5 if tested at one angle and 7 if tested from another angle.
Since the Mohs scale, there have been several other more controlled tests developed including Brinell, Knoop, Rockwell, Shore and Vickers, where a tool is scratched into a surface to determine the hardness, taking into account the force used and the size and depth of the impression left by the tool. The Mohs scale, however, in its simplicity, continues to be the most used test for hardness. It is important to remember that these tests will only tell you how hard a material is and will not indicate how tough or strong a material is.

According to the Mohs scale, only diamond, the hardest of gemstones, is 10 and at around 2 is amber, the softest.
Here is the Mohs scale hardness for some of the most common gemstones:
Diamond - 10
Ruby and Sapphire - 9
Topaz and spinel - 8
Emerald and aquamarine - 7.5 to 8
Amethyst, smoky quartz, tourmaline and citrine - 7
Tanzanite, tsavorite and peridot - 6.5 to 7
Moonstone and jade - 6 to 6.5
Opal - 5.5 to 6.5
Turquoise - 5 to 6
Pearl - 2.5 to 4.5
Amber 2 to 2.5

All gemstones can be set into jewellery for example take a look at our amber engagement rings

Jewellery metals can also be ranked on the Mohs scale of hardness, although they possess many other properties which make them very durable and long-lasting unlike gemstones:

Palladium - 4.5
Platinum - 3.5
Silver and gold - 2.5 to 3
Here is a ring that caught my eye which beautifully incorporates the hardest and one of the softest gemstones! It combines the colours of autumn with the warmth of white gold and the sparkle of diamond.