Hardness (the ability to resist scratching) is one of the key qualities of gem stones and can be measured by the Mohs scale, created in 1822 by the German mineralogist Friedrich Mohs.
Mohs chose 10 commonly available minerals and gave each one a value between 1 and 10. As many of you will know, diamond is the hardest material and therefore sits at the top of the table at number 10. The softest mineral on the scale is talc which is positioned at number 1. Each mineral on the scale can be scratched by those above it and will scratch those below it. Minerals of the same hardness will not scratch each other.
While Corundum (sapphires and rubies to you and me) appear at number 9, diamonds at number 10 are, in fact, four times harder than these minerals so there is a big difference between the two groups of minerals.
While the Mohs scale is a useful start point, hardness should not be taken as a measure of durability. For example, even though emeralds have a hardness level of 7.5, they often contain tiny fractures means they can break when knocked. This is worth bearing in mind when purchasing jewellery, especially if you are selecting something like an engagement ring that will be worn regularly and will be subject to a lot of wear and tear. For this reason, I would recommend wearing an emerald ring for special occasions only.
Here is the Moh scale, showing the 10 original minerals that Moh used in descending order of hardness. I have included examples of other common gemstones at each level in brackets.
9 Corundum – Rubies and Sapphires
8 Topaz (Alexandrite; Chrysoberyl)
7 Quartz (Amethyst; Citrine)
6 Feldspar (Peridot; Onyx; Jasper)
5 Apatite (Opal; Turquoise; Lapis Lazuli)
3 Calcite (Coral)
2 Gypsum (Pearl; Amber)
For more information on Mohs scale ratings, check out http://www.gemselect.com/gem-info/gem-hardness-info.php