Gemstones on Mohs Hardness Scale
Although there are many methods and systems used to gauge a material’s hardness, the most common of these is the Mohs Hardness Scale. Mohs Hardness Scale, a system founded by Friedrich Mohs in 1812, is a type of chart used to test and contrast the hardness of different minerals. 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. In other words, it allows our custom jewelry designer in Seattle and Bellevue to rank the hardness of a mineral by its ability (or inability) to scratch another known mineral.
Mohs Hardness Scale Simplified
Mohs Hardness Scale is based on 10 minerals, each of which is assigned an arbitrary value from 1 to 10. These minerals, along with their assigned value are:
| Mineral | Mohs Hardness |
|---|---|
| Talc | 1 |
| Gypsum | 2 |
| Calcite | 3 |
| Fluorite | 4 |
| Apatite | 5 |
| Orthoclase | 6 |
| Quartz | 7 |
| Topaz | 8 |
| Corundum | 9 |
| Diamond | 10 |
| Common Gemstones | Mohs Hardness |
|---|---|
| Turqoise | 5 |
| Opal | 5.5 |
| Moonstone | 6 |
| Garnet, Zircon, Tanzanite, Peridot, Bloodstone | 6.5 |
| Quartz, Amethyst, Citrine | 7 |
| Aquamarine, Tourmaline, Emerald, Pearls | 7.5 |
| Topaz, Spinel | 8 |
| Alexandrite | 8.5 |
| Corundum, Ruby, Sapphire | 9 |
| Diamond | 10 |
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.
Using Mohs Hardness Scale
While Mohs Hardness Scale outlines the hardness or scratch resistance of ten common minerals, its true usefulness is in measuring the hardness of countless of other minerals and materials, almost all of which fall somewhere in between talc (1) and diamond (10). Gold, one of the most common minerals used by jewelers, generally ranks between 2.5 and 2.9 on Mohs Hardness Scale.
This number can be consistently ascertained by the ability of a particular sample of gold to scratch or leave an abrasion on gypsum (2), while being vulnerable to scratches when coming into contact with calcite (3) and all harder materials. Below we will take another look at Mohs Hardness Scale, as well as some common materials and how they compare in terms of hardness.
| Mineral | Hardness Ranking | Common Materials | Observations |
|---|---|---|---|
| Talc | 1 | Very soft (almost greasy). Can be scratched by a fingernail. | |
| Gypsum | 2 | ||
| Fingernail (2.2) | Fingernails can scratch gypsum but will be scratched by a penny | ||
| Calcite | 3 | ||
| Copper Penny & Dolomite (3.5) | Both will scratch calcite but are scratched by fluorite | ||
| Flourite | 4 | Can easily be scratched by a knife | |
| Apatite | 5 | ||
| Pocket Knife & Glass (approx. 5.5) | Glass (with great difficulty) can be scratched by a pocket knife | ||
| Orthoclase | 6 | Cannot be scratched by a pocket knife | |
| Quartz | 7 | Can scratch glass easily. | |
| Garnet Paper (7.5) | |||
| Topaz | 8 | ||
| Corundum | 9 | Sandpaper | Common sandpaper uses corundum and has a hardness value of 9 |
| Diamond | 10 | Used as a glass cutter |
It is important to remember that the Mohs Hardness Scale, while certainly the most common measuring tool, is just one of the ways to identify a material and gauge its hardness and scratch resistance. The rock type, as well as its color, cleavage, luster and crystalline form, must also be taken into account, in addition to the material’s ranking on the scale.