The hardness of a gemstone is an important data: it can for example influence the way it can be worked in jewelry. It is measured using the Mohs scale, established by Friedrich Mohs in 1812. This scale measures the scratch resistance of stones, in a comparative way.
Mohs selected ten reference minerals: number 1 is the softest and number 10 is the hardest. Thus, gypsum (number 2) scratches talc (number 1), calcite (number 3) scratches gypsum (number 2)... until diamond (number 10, the hardest mineral) which scratches corundum (number 9).
Two minerals of the same hardness can also scratch each other, but with more difficulty. Thus, only diamond can scratch diamond. Half scales are also used: chrysoberyl with a hardness of 8 ½, for example, which is harder than topaz but not as hard as corundum (sapphire and ruby).
Mohs scale, source (French / English)