A crappy chair can screw up someone's back, so logically, buying a good chair is justified and it gives mental happiness. Same for monitor (crappy monitor can

A crappy chair can screw up someone's back, so logically, buying a good chair is justified and it gives mental happiness. Same for monitor (crappy monitor can screw one's eyes, give headaches etc). So it is all logical. what real use does a piece of rock (which is what a diamond is) have? How can it possibly give mental happiness? I'm not sure I understand