Diamonds aren’t “needed” on anything outside of a few industrial applications, truthfully. But, they’re beautiful and so they’re wanted. To each his own, if you find it excessive then to you it is. Don’t buy any. Kinda cool how that works, isn’t it?
Apart from their cost, diamonds are in fact beautiful. Their dirty little trick is that they have a higher refractive index than just about any other crystalline substance, which is what makes them “sparkle”. And the covalent bond holding the lattice structure together essentially makes a whole diamond one gigantic molecule, which is what makes them so hard. I’d like to have a 10 carat one just to look at and another to sell.