Created by the German mineralogist Friedrich Mohs in 1832, the Mohs scale is used to measure the hardness of minerals. Mohs based his scale on ten minerals that were available to him at the time. The hardest naturally occurring mineral Diamond is at the top of the scale and therefore is rated 10.
Today, it is possible to measure the scratch hardness of minerals more accurately by using an instrument invented in 1896 called a Sclerometer. However, the Mohs scale is still widely adopted around the globe as an accurate measurement, and is therefore still used to measure the hardness of gemstone.
Whilst many scales and measurements in the world can be quite complex, the Mohs scale is incredibly simple. Imagine you have just discovered a new mineral: if it scratches Apatite (5 on the scale) but does not scratch Orthoclase (6 on the scale), your new discovery registers a hardness of 5.5 on the Mohs scale.
However, due to the way the scale works, it simply ranks gems in order of their hardness. What it does not do is tell you the difference in hardness, i.e. how much harder a gem that records 8 is compared to one which only reaches 6. For example, Diamonds are presently the hardest known mineral (therefore being 10 on the scale), and are approximately four times harder than Sapphire which registers 9 on the Mohs scale and 30 times harder than Apatite at number 5.