We all know that diamonds are the hardest substance on earth, but how do we know this? It’s probably because we heard it somewhere. Most likely we haven’t put this well known fact to the test by using our diamond rings to scratch other gems and metals, though lots of folks like to try the diamond versus glass test.
This scratch test is one of the main ways of looking at the hardness of a gemstone. Another is cutting hardness. But first let’s look at scratch, since it is the one most commonly referenced. This type of hardness, which is measured on the Mohs’ Scale, is a numerical value from 1 to 10, based upon the relative hardness of one gem to another.
In 1825, Viennese mineralogist, Friedrich Mohs, published his book, Treatise on Mineralogy: Or, The Natural History of the Mineral Kingdom in which he described the scratch test and set the standards on his Mohs’ Scale. Here are the values he came up with.
| Talc | 1 |
| Gypsum | 2 |
| Calcite | 3 |
| Fluorite | 4 |
| Apatite | 5 |
| Feldspar | 6 |
| Quartz | 7 |
| Topaz | 8 |
| Corundum | 9 |
| Diamond | 10 |
Take any gemstone and it will fall somewhere on this scale from 1 to 10. For example, aquamarine has a Mohs’ Scale value of 7.5-8, which means that it will scratch quartz, but that corundum, which includes rubies and sapphires will scratch it.
The fact that aquamarine is harder than quartz makes it suitable for everyday. In fact any stone that is harder than quartz fits that bill since quartz is the main component of dust. What all this comes down to is that if you do have a gem with a Mohs’ Scale value less than 7, you need to take extra care that it won’t become scratched and damaged
Whereas this measure of hardness is important to the consumer, the other measure of hardness, cutting hardness, is important to the lapidary, that person who transforms the rough into a shaped and polished stone. Cutting hardness relates more to the internal structure of the stone, whereas you might say that scratch hardness is skin deep.
When buying your finished stone, you need not worry so much about the cutting harness, but you will want some idea about your stone’s scratch resistance, so remember to ask your jeweler if he doesn’t volunteer the information. And if he does, you will know what he’s talking about.
