Mohs Hardness Scale

The scale of hardness which is used universally for Gemmology and Mineralogy was devised by the German mineralogist, Friedrich Mohs in 1812. This scale is a comparative one, and is based on the principle that any substance with a given hardness value or number, will scratch another substance having a lower number, and will in turn be scratched by one having a higher number.

The scale, which can best be described as ordinal rather than linear or logarithmic, provides a rough measure of a mineral’s hardness based on its smooth surface resistance to scratching and abrasion.

The Mohs Table of Hardness (Simplified)

 (Progressing from Softest to Hardest)

1. Talc
2. Gypsum
3. Calcite
4. Fluorite
5. Apatite
6. Feldspar
7. Quartz
8. Topaz
9. Corundum (Sapphire)
10. Diamond

Table of Hardness of Gem Minerals

The following is a non-comprehensive list of Gem Minerals (From hard to soft)

• Diamond – 10
• Corundum (Ruby & Sapphire) – 9
• Chrysoberyl – 8.5
• Spinel, Topaz – 8
• Beryl (Emerald, Aquamarine), Tourmaline – 7.5
• Quartz, Amethyst, Citrine – 7
• Garnet, Zircon, Tanzanite, Peridot, Bloodstone – 6.5
• Moonstone – 6
• Opal, Obsidian – 5.5
• Apatite, Turquoise – 5
• Fluorite – 4
• Calcite – 3
• Gypsum – 2
• Talc – 1

When interpreting Mohs Hardness Scale, it is common to assume that a diamond, for instance, the hardest mineral listed on the scale, is ten times harder than talc, which is the lowest. This is NOT true, but is a common mistake. So, when interpreting the Mohs Hardness Scale, it’s important to keep in mind that the scale is merely ordinal rather than linear. For example, corundum, with a value of 9 on Mohs Scale of Hardness, is twice as hard as topaz, with a value of 8, while a diamond is nearly four times as hard as corundum.