Originally Posted by mafoofan
I have no idea what the scientific explanation is. But sapphires are much, much harder. Rubies can crack and chip much more easily. The color fades too. They turn pink over time.
Diamonds are tetrahedrally bonded carbon atoms packed into a very dense FCC structure, this crystal lattice structure contributes to the extreme hardness of diamond. (Mohs scale = 10)
Unlike diamonds (purely carbon), sapphires and rubies are both derivatives of corundum (a molecule made of Aluminum and Oxygen atoms). Sapphire and rubies are formed when impurities are introduced to the corundum mineral structure. Theoretically (i.e. on a molecular scale) both sapphires and rubies should have the same hardness (Mohs scale = 9); But since sapphires and rubies are naturally formed crystals, their physical properties vary at the macroscopic level. On average, sapphires and rubies should be of the same hardness.
Emeralds are an entirely different story.