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. Founded by Friedrich Mohs in 1812, it is a chart used to test and compare the hardness of different minerals. The scale is best described as ordinal rather than linear or logarithmic, providing a rough measure of a mineral’s hardness based on resistance to scratching and abrasion. In practical terms, it allows us to rank 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 assigned an arbitrary value from 1 to 10. These minerals, along with their assigned values, 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 and Typical Hardness
| Common Gemstones | Mohs Hardness |
|---|---|
| Turquoise | 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 (10) is ten times harder than talc (1). This is not true. The scale is ordinal, not linear. For example, corundum (9) is roughly twice as hard as topaz (8), while diamond is nearly four times as hard as corundum.
Using Mohs Hardness Scale
While Mohs Hardness Scale outlines the scratch resistance of ten reference minerals, its usefulness is in estimating the hardness of countless other materials that fall somewhere between talc (1) and diamond (10). Gold, one of the most common metals used in jewelry, generally ranks between 2.5 and 2.9 on Mohs Hardness Scale.
This can be estimated by whether a particular sample of gold can scratch gypsum (2), while still being vulnerable to scratches from calcite (3) and harder materials. Below is another look at Mohs Hardness Scale with common materials and observations.
| 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. |
| Fluorite | 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 | Garnet paper (7.5) | Can scratch glass easily. |
| 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 Mohs Hardness Scale is only one way to identify a material and estimate scratch resistance. Rock type, color, cleavage, luster, and crystalline form should also be considered alongside a material’s ranking on the scale.