Gold is just a metal and diamond is just a stone, why not we call their value too as overpriced ? Unlike crypto currency, gold/Diamonds are being valued by every part of this world ? All these people are so childish to value just a metal or a stone ?
Gold is not just a metal, it is very useful because it has some excellent chemical properties. Can be used for jewelry or to create tools or electronics.
Diamonds are simply beautiful when used as decoration and they have industrial use because they are very hard and are good abrasives.
Usefulness and scarcity = high price.