Small-time jeweler and gem dealer also here. I’d like to qualify that hardness of a stone only refers to resistance to abrasion (scratching), and is distinct from tenacity, the resistance to breakage and deformation. Diamond, for instance, has perfect cleavage (planes of weakness in the crystal) in 4 directions (like the top of a pyramid) so, despite its extreme hardness, it’s actually vulnerable to breakage relative to sapphire, which isn’t as hard. To be clear, it still has good tenacity, just not excellent tenacity.