Diamonds are one of the hardest things known to man. So as to which one between a diamond or a ruby being breakable, it would be a ruby. Diamonds are breakable and were originally faceted along cleavage planes with hand tools. The mineral diamond is rated as the hardest mineral on the Mohs Scale of mineral hardness. Ruby (corundum) is rated as the next hardest. The Mohs rating is a relative scale that measures the ability of one mineral to scratch another mineral.