Because diamonds aren’t rare.

Common misconception, I know, but let’s get more specific.

These are diamonds:

Not what you were expecting? Well, these are industrial diamonds, useful as an abrasive, because they’re so hard, but for little else. We can now produce them artificially, for around seven bucks a pound. They’re routinely used to coat drill bits and grinding discs. They’re really no big deal.

Now, when you think about diamonds, I assume you’re thinking about gem-quality diamonds:

Those still aren’t quite as rare as the diamond industry would have you believe, but they are less common. In order to work as gems, you need a much larger, more consistent diamond crystal. Those don’t occur in nature as often, as they’re harder to make synthetically (but not impossible).

The simple answer to the question is that we are so far from being able to make it economically worth it that it’s not really worth thinking about. I mean, as things stand, we don’t have any way of getting a rocket into Saturn’s atmosphere and back out, even in theory.

But even if we did, what you’d end up with is bags of diamond grit that look like gravel, and would be worth more than gravel, but not by that much. It’s already far cheaper to make diamonds from charcoal right here on earth, and that’s not likely to ever change.

The only reason people think they’re valuable is that an economic cartel has established artificial pricing by creating artificial scarcity.

Fiamonds are more common than tanzanite. Diamonds would be a minor semiprecious stone of little interest without a powerful cartel and a ton of extremely sophisticated marketing.