One of the characteristics of crystals is that they have cleavage. Cleavage is the tendency of a crystal to break cleanly along distinct planes. Since most gemstones are crystals, cleavage is an issue that gem buyers as well as jewelers and gem cutters need to take into account.
Minerals can have from one to five cleavage planes, and each cleavage plane has a grade or rating, indicating the relative ease with which the crystal can be cleaved. Cleavage is graded as perfect, good, indistinct or "none" (in the case where a gemstone has no cleavage at all).
The following chart lists the cleavage ratings for many well known gemstones. You'll note that different members of the same species or family, such as quartz or beryl, have the same cleavage. That's because members of the same species have the same crystal structure.