Why diamonds? Diamonds are a 10 on the Mohs scale, a scale used to gauge the hardness of gems and minerals. Corundum (more commonly known as ruby and sapphire) is a 9, which would lead one to believe that it’s nearly as hard as a diamond. But what most people don’t realize is that while corundum is very hard, the difference in hardness and durability between a diamond and any other gemstone or simulant is quite significant. Looking at the chart gives new meaning to the saying “Diamond are Forever”.