How gemstone hardness measured

How gemstone hardness measured

by Binashaji

Diamonds measure 10 on the MOHS scale and Aquamarine measures 7 whereas Pearls can measure between 3 and 4 on the MOHS scale. MOHS is a term oft used in Gemstone Jewelry, but what exactly does MOHS mean and signify?

The MOHS scale was formulated in the 1800’s by a German mineralogist (Geologist) named Friedrich Mohs. As a Geologist, he spent over a decade studying and experimenting with various minerals and gemstones. Friedrich Mohs created an easy to remember scale by which one could easily discover if a mineral was delicate or hard. The resultant MOHS scale makes it possible to gauge the scratch ability of a mineral or gemstone. He took 10 minerals and gave each one a number from 1 to10 depending on its hardness as compared to the minerals above and below it. The MOHS scale is given below:

The symbol for MOHS is HM.

Mohs Scale of Hardness
1Talc (softest)
7Quartz (Amethyst)
6.5 – 7.5Garnet
7.5Beryl (Emerald)
9Corundum (Ruby and Sapphire)
10Diamond (hardest material known to man)

Although Friedrich Mohs designed this model for minerals in general, it is not used scientifically in laboratories because it is not an exact measurement. The MOHS scale is only a comparative scale. The degree of the hardness is only to check if a gem can be scratched by another gem that is above or below it. For eg. Garnets and Amethyst are both rated as 7 on the MOHS scale and it means that both can scratch each other if they are handled without care or placed with each other. But if a Garnet is placed with a stone like opal which is 6 HM then the opal can be scratched by the Garnet. Alternatively if the Garnet is placed with a Ruby (hardness of 9) then the Garnet can get damaged but not the Ruby. So if new , unknown gem is discovered and its hardness is not known , it can be rubbed against various known gems. Depending on which material leaves scratches on it, it hardness on the MOHS scale can be arrived at with relative ease.

The MOHS scale only determines the ability of the gemstone to resist scratches. It is not a measurement of the strength of the stone. A high MOHS rating does not necessarily mean the material is strong. For example a Diamond is 10 HM, which means that it measures 10 on the MOHS scale. Diamonds are the only substance with a rating of 10 on the MOHS Scale. They are the hardest gemstones known to man and Diamond bits are used to cut glass, metal and other diamonds also. Although Diamonds cannot be scratched easily, they are brittle. A light blow and a diamond can crack and break.

There are several other exact scientific methods to arrive at the exact hardness of any material. It is not possible to decide that a stone which has a hardness of 3 is exactly half as hard as a stone which measures 6 on the MOHS scale. For eg. Corals measure 3.5 and an Amethyst is 7on the MOHS scale. But accurate scientific tests have shown that the hardness of the Amethyst is more than 20 times the coral. The numbers accorded to the stones on the MOHS scale are only for comparison and they are not proportionate with each other.

But the MOHS scale has been a boon to the Jewellery industry and specially for the Gemstone buyers. This easy to remember scale makes it very easy to maintain and care for Gemstone Jewellery. This scale helps even a layman understand about how Jewellery should be maintained and stored. Storing Aquamarines with Pearls can be disastrous. The Aquamarine will scratch the delicate pearls leaving ungainly scratches which can be felt if a finger is rubbed gently across the Pearls. These scratches will even show up when light falls on the earrings or necklace, ruining the classiness of the Pearls.

When buying Jewellery, just ask your Jeweler what is the MOHS rating of the gemstone. It is much easier to store and maintain Jewellery when you have an idea about the MOHS rating of piece you have bought. Corals, Pearls, Amber all are delicate and need to be used with care. As a rule, if your gem does not measure at least 7 on the MOHS scale, it is a delicate stone and can be damaged very easily. So extra care should be taken while wearing these ornaments.

The MOHS rating can help determine the usage of the Jewellery too! For everyday wear, Diamonds are the best – their hardness is 10 and they do not get scratched easily. Rubies, Emeralds also are great for people with active lifestyles since they measure 9 on the MOHS scale and have a greater resistance.

If you love wearing Gemstone Jewellery, get to know the MOHS scale to help you understand and care for your precious pieces. Although the scale is not an accurate measure and can only be used for comparisons, it is great to care for your precious ones !