The Mohs hardness scale is a measure of a mineral's resistance to scratching or being sawed apart, not of the tenacity, which is whether the mineral can be shattered or split in two by impact. For example, diamond is the hardest substance known to man, but it is somewhat brittle and will split along planes where the bonds in its atomic structure are relatively weak. A good gemstone should have a hardness of at least 6, and even that might get scratched in everyday wear.