The scientific definition of hardness is the ability to resist scratching, nothing more. The Mohs scale of mineral hardness was created by the German mineralogist, Friedrich Mohs in 1822. It is used to measure the relative hardness or scratch resistance of various minerals on a scale from one to ten.
On Moh’s scale, a diamond is rated 10, meaning that it can scratch all the other gems rated below it. To give an example of how the scale works, topaz can scratch all gems with a lower rating, but can be scratched by diamond, sapphire & ruby.
Below is a Gemstone Hardness Scale
9 Sapphire, Ruby
7.5-8 Aquamarine, Emerald, Morganite, Peridot
7 Quartz, Garnet, Citrine, Tourmaline, Onyx
6 Opal, Jade
The hardness of each gemstone is very similar at the low end of the scale, however the hardness curve climbs sharply at the high end. Diamond (10) is about 4 times harder than sapphire & ruby (9), but sapphire & ruby is only 2 times harder than topaz (8). Topaz is twice as hard as quartz (7), but quartz is only about 25% harder than apatite (5).
Many people mistakenly believe that hardness is the key to gemstone durability but there is more to durability than just hardness. For example, a diamond will shatter into multiple pieces if hit with a hammer because a diamond has perfect cleavage, meaning it can be split with a single blow.
The same is true for topaz.
Ruby and sapphire, while not as hard as diamond, has no cleavage, a property it shares with spinel and quartz.
In practical terms, gems with a hardness of 7 or higher usually indicates that gem is hard enough for normal jewellery use. This is a guideline only because Pearls and opals are very popular gems but are well below 7.