The ability of a mineral or gemstone to resist breakage. Jadeite is the toughest gemstone due to an interlocking molecular structure that helps to minimize its weak points. Diamond is not the toughest gem material because it does have certain planes of Cleavage that make it possible to separate or cleave a diamond with the right force and angle.
This is not to be confused with Hardness, which is a measure of a gem’s scratch resistance. Diamond has no comparable to it’s hardness. Corundum is second, but is far far behind the hardness of a diamond on the Mohs Scale.