Adding to what that other person said, not all pearls are perfectly round, and even rarer are black pearls. Because of that, they can be used as a symbol of status. Its a bit more interesting in case of diamonds (and im a bit more sure in what I know, but its a similar process), because they have multiple fascinating uses (diamond saws, pretty sure drill heads too, lasers), but the status aspect really dominates what really makes them expensive. There are a lot of diamonds out there, but jewelry corporations want you to care only about the natural ones with little imperfections (synthetic diamonds have a lot less imperfections, but its harder to control the supply of those, so you are told they suck). So a decent part of it is corporations upholding either tradition or old marketing, whichever you prefer and like in many cases, the intuitive answer is "look pretty" but the real one is politics