Gemstone Hardness and The Moh’s Scale

Gemstones are rated by levels of hardness using the Mohs' scale. This method was developed in 1812 by the German mineralogist Friedrich Mohs. He used ten commonly available minerals and classified them according to their hardness, and their ability to scratch and be scratched by other minerals.

Diamond, which is the hardest naturally occurring substance, is highest on the scale, and talc, which can be
scratched with a fingernail, is at the bottom. The position of a substance on the scale is determined by finding the hardest material that the given substance can scratch, and/or the softest material that can scratch the given substance. The scale is not even, as can be seen by the notes beside the minerals on the scale.

The Mohs' Scale
10 - Diamond (four times as hard as Corundum)
9 -  Corundum (Ruby & Sapphire, twice as hard as Topaz)
8 - Topaz, Emerald, Aquamarine
7 - Quartz
6 - Orthoclase Feldspar
5 - Apatite
4 - Flourite (slightly harder than calcite)
3 - Calcite
2 - Gypsum
1 - Talc

Some examples of common substances that are rated on the Mohs' Scale are:
7-8        Steel
6.5        Glass
6           Titanium
3           Copper Penny
2.5-3    Gold and Silver
2.5        Fingernail

Common gemstones are rated on the scale as follows (from hardest to softest):
10           Diamond
9              Ruby, Sapphire
8.5          Alexandrite
7.5 - 8   Aquamarine, Emerald, Topaz
7 - 7.5   Garnet, Tourmaline
7              Onyx
6.5 - 7   Amethyst, Citrine, Tanzanite
6.5         Jadite, Peridot
6             Jade, Opal
3             Pearl

The hardness of minerals is important to jewelry artisans. They would not want to set a very hard stone next to a soft substance that can be scratched easily. Also softer stones are typically included in settings that are designed to protect the stone from wear.  Generally valuable gemstones have a hardness of at least 7. Stones that are rated below 7 are likely to be damaged easily in jewelry that is worn everyday.  The Moh's scale is a universal measurement scale that can be relied upon by jewelers to assist functional elements of jewelry design.

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