The hardness of a stone is one of its key properties in determining its appropriateness for different types of jewelry items. Hardness refers to its resistance to scratching or how the surface responds to a sharp object. This property helps to determine the durability of a stone, and whether it is appropriate to be used as a ring (which may get hit with surfaces more often) or only earrings / pendants.
The scale is not linear, the hardness is more of a relative reference point between each stone; for example, although diamonds are 10 and corundum (Rubies and Sapphires) are a 9, the difference in hardness is actually many times.A more subtle point, but equally important, is that a stone which is harder is able to take on polish much better. As a result of that the stones have better luster and look more beautiful in comparison to stones which may be softer and don’t have the same luster.
Not mentioned in the graph from GIA, but Beryls (such as Emerald, Aquamarine, Morganite and Heliodor) as well as Spinels are an 8 on the hardness scale.