Diamond being the hardest material there is gets read as a guarantee against damage. It isn't one. Hardness measures resistance to scratching. It says nothing about resistance to breaking, and that's a separate property called toughness.
On toughness, diamond only rates good. Sapphire and ruby rate excellent. The reason is cleavage: diamond has four directions where its crystal structure will split cleanly under a sharp knock. Sapphire and ruby have none.
It shows up most at thin girdles and in pointed cuts, pear and marquise in particular, where there's less stone around the point to absorb an impact.
A diamond won't scratch under normal wear. A hard enough knock at the wrong angle is a separate risk, and it has nothing to do with the Mohs scale.