If it's just that "they tell us they are" and "it's just a rock" - then why don't they make actual rocks that expensive? If it's all about what "they" tell us to believe is expensive? And why do different gems have different prices, if they can simply decide what price we should accept? Then they could just give every gem the highest price.

And why don't they make every diamond equally expensive? Why are the many impure diamonds worth less? If they control the price, they could set the same price for these and make people pay it.

People like beautiful things. Beauty exists. That which glitters - how slandered and belittled it is to like this. Yet all over the Earth people have liked gems for their beauty. People were surrounded by the same boring brown and grey in their daily lives and wanted something different. We might understand this less today because we have a lot more color and pleasing things around us.

Everything in our homes is made with an eye to aesthetics. Things are painted. Forks and knives have handles shaped in a way that isn't needed for practicality. Toothbrushes too - razors, desks, chairs, that computer on the desk, everything. Decorated book covers. We don't "need" wallpaper. Except when you realize the importance of aesthetics. The explanation for the value we place in the aesthetic is long, and there are actually a couple of different reasons for the biological and other things, but suffice to say that it is of great importance to us. Again, when we see the same in cultures all over the world - an appreciation for aesthetics, for adding color etc - develop separately, there is a meaning to it. Same as music. We don't "need" music - unless we realize how it affects our brains.