Toughness Scale A mineral’s toughness is its ability to resist being fractured. Toughness means resistance to breaking, chipping or cracking in general, how well a stone holds up under mechanical stress, such as impact from a fall. Toughness Scale: Exceptional, Excellent, Good, Fair, or Poor. Even though diamond is the hardest mineral it can chip and fracture in normal day to day wear and tear. Diamond is formed in the cubic crystal system and has 4 perfect cleavage directions. A cleavage direction is the weakness plane in the molecular crystal of a diamond. Hence, an experience diamond cutter will look for the cleavage plane to cleave a big diamond. The toughness rating of diamond is only ‘good’ while that of Jadeite Jade and Nephrite Jade have a toughness graded as Exceptional.