Again, hardness is the ability to resist scratching and nothing more. Just because a gem is hard does not mean it will wear well. There are a number of factors that effect how well a gem will wear. Technically, cleavage has to do with how strongly the molecules bind to each other. To put it in layman's terms, it is much like the grain of wood.
Many gems have cleavage planes.
Cleavage planes are not the same in all minerals. They are defined as perfect, good, fair or poor, depending on how easily the mineral will separate along the plane. You may have seen an old movie where a diamond cutter is sweating over cleaving a large, valuable diamond crystal. Cautiously he places the chisel on the crystal, then he gives it a brisk and carefully measured blow. If he does it right, he has two perfect pieces that can be cut into fabulous gems.