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)
- Talc
- Gypsum
- Calcite
- Fluorite
- Apatite
- Feldspar
- Quartz
- Topaz
- Corundum (Sapphire)
- 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.