Mohs Scale of Hardness
What is the Mohs scale?
Friedrich Mohs was the German mineralogist who created a scale of mineral hardness that is still used today as a standard to measure the hardness of gemstones and other minerals. He lived from 1773 to 1839 and he selected the ten minerals based on what was commonly available at the time. His scale is based on the scratch resistance of ten minerals that are readily available and gives the hardness on a scale of 1 to 10 where diamonds are the hardest material and therefore score a 10.The hardness of a material is measured on the scale by finding the hardest material the given material can scratch. For example, since a ruby can be scratched by a diamond (a Mohs 10), but cannot be scratched by a yellow topaz (a Mohs 8), it's Mohs hardness falls between a 10 and an 8, making the hardness of a ruby around a 9.
The Mohs scale is depicted below. While the scale is very useful, it cannot be used as a scale of absolute hardness (that measured by a sclerometer).
| Mineral | Scale |
|---|---|
| Talc (talcum powder) | 1 |
| Gypsum (plaster of Paris) | 2 |
| Calcite (limestone and shells) | 3 |
| Fluorite (related to fluoride) (also platinum) | 4 |
| Apatite (also cobalt has this hardness) | 5 |
| Orthoclase (feldspar) | 6 |
| Quartz (also glass) | 7 |
| Topaz (emerald is a variety of beryl) | 8 |
| Corundum (rubies and sapphires) | 9 |
| Diamond (also boron) | 10 |