The hardness of gemstones is a very poorly understood property. The word "hardness" has a very specific scientific meaning in gemology that differs greatly from its everyday use. The scientific definition of hardness is the ability to resist scratches, nothing more. If you ask most people, they will tell you that feathers are soft and glass is hard. In the world of gemology, however, glass is rather soft. A variety of what we, gemologists, consider hard materials, could easily scratch it.
Hardness depends on the bonds that hold atoms together in a crystalline structure. This bond is evident in the ease with which layers of atoms from a surface can be separated by applying pressure with a sample of another material. If the second material is harder than the first, it will leave a groove, or a scratch. This represents the breaking of millions of atomic bonds on a microscopic scale. The hardness of a mineral is, more precisely, its "scratchability". Each mineral can be ranked based on the other minerals it can scratch.