Diamond is the hardest substance on earth and so naturally Friedrich Mohs based his hardness scale on just that.
9) corundum (ruby/sapphire)
The divisions were chosen arbitrarily and are unequal. For example the difference in hardness between 8 and 9 is much greater than the difference between any of the lower numbers. The difference in hardness is greater between 9 and 10 than between 1 and 9. Needless to say diamond is an extremely hard substance capable of resisting scratching by any other substance on earth except for diamond itself.