Mohs Hardness Scale

October 18, 2020

By Yagé Enigmus

For ages humans have observed that some materials were harder than others and could be used to scratch or grind down softer materials. This observation would have inspired primitive gem cutting practices in the ancient world now known as ‘bruting’, and early documentation of this knowledge appears in On Stones by Theophrastus (circa 300 B.C.E.) and Naturalis Historia by Pliny the Elder (circa 77 C.E.).

In 1812 Friedrich Mohs, a german mineralogist and geologist, developed a qualitative scale for categorizing materials based on their relative hardness. This ‘Mohs Hardness Scale’ has since become an important tool for identifying minerals and understanding certain aspects of their durability. The scale is broken down into ten main categories, with 10 being the most scratch resistant and 1 being the least scratch resistant, and each having a best example or ‘index mineral’ to describe the category. When making gemstone jewellery, one must take care to choose a stone with a high enough Mohs hardness to withstand the abrasions that the piece may be subjected to. For example, stones with a high Mohs hardness are best for engagement rings as they will likely receive abrasions while being worn, especially since they will be worn every day; for this reason sapphire engagement rings are always a safe choice as sapphires rank at 9 on the Mohs hardness scale.

Mohs Hardness	Materials [Index Minerals Bolded]
1	Talc
2	Gypsum (Selenite), Cerussite, Mica (Lepidolite, Muscovite, Biotite)
2-2.5	Chlorite (Seraphinite)
2.5	Pearl
1-3	Amber
2-3	Gold, Silver
3	Calcite, Copper
3.5	Coral, Platinum
3.5-4	Malachite, Rhodochrosite, Sphalerite
4	Fluorite, Iron
3.5-4.5	Ammolite
4-4.5	Pectolite (Larimar), Common Steel
4.5-5	Kyanite [along one axis]
5	Apatite, Obsidian
5-5.5	Lapis Lazuli, Titanite (Sphene), Scolecite
5.5	Common Glass
5-6	Turquoise, Charoite
5.5-6	Scapolite, Sodalite
6	Orthoclase Feldspar (Classic Moonstone), Titanium, Rhodium
5.5-6.5	Opal, Rhodonite
6-6.5	Pyrite, Plagioclase Feldspar (Labradorite, Rainbow Moonstone, Sunstone), Microcline Feldspar (Amazonite), Prehnite, Nephrite
5.5-7	Tektites (Moldavite, Saffordite, Columbianite, Libyan Desert Glass)
6.5-7	Zoisite (Tanzanite, Thulite), Olivine (Peridot), Grossular Garnet (Hessonite, Tsavorite), Andradite Garnet (Demantoid, Topazolite), Uvarovite Garnet, Jadeite, Spodumene (Kunzite, Hiddenite), Chalcedony (Carnelian, Agate, Chrysoprase), Tiger’s Eye, Jasper, Kyanite [along one axis]
7	Quartz (Amethyst, Citrine, Prasiolite, Ametrine), Fused Silica Glass
7-7.5	Pyrope Garnet, Almandine Garnet, Rhodolite Garnet, Spessartite Garnet, Grandidierite, Iolite, Tourmaline (Elbaite, Dravite, Liddicoatite, Schorl), Danburite
7.5	Emerald, Zircon
7.5-8	Aquamarine, Heliodor, Morganite, Goshenite, Spinel, Hardened Steel, Tungsten
8	Topaz
8.5	Chrysoberyl (Alexandrite, Cymophane)
9	Corundum (Ruby, Sapphire), Silicon Carbide
10	Diamond

This scale is an ordinal scale, meaning the categories are relative to one another rather than absolute. This means that if something has a Mohs hardness of 4, it is able to scratch everything softer than 4 and can be scratched by itself and everything harder than 4.

Conversely, a hardness of 4 is neither twice as hard as a hardness of 2, nor half as hard as a hardness of 8. Measurements for the absolute hardness of materials do exist; one way this is done is using a sclerometer, which is a specialized laboratory device that uses a tiny diamond to scratch a material with a fixed amount of force and measure the size of the scratch left behind.

Mohs Hardness	Index Mineral	Absolute Hardness [Sclerometer]
1	Talc	1
2	Gypsum	2
3	Calcite	14
4	Fluorite	21
5	Apatite	48
6	Orthoclase	72
7	Quartz	100
8	Topaz	200
9	Corundum	400
10	Diamond	1500