Diamond prices are down 60% since a 2011 high, and they are still falling. It's not all down to lab-grown diamonds, demand is down too, especially in China.
No one can lab-grow gold yet, so its rarity and scarcity protect its value, but that will end too. It's just a question of when. China launched an asteroid touch-down mission this week, which will make it the 4th country/region to do so, after Europe, the US & Japan.
How soon will it be feasible to mine asteroids? Who knows, but a breakthrough in space propulsion might mean the prospect happens quickly when it does. It's possible gold has twenty years or less of being high value left.
The $80 Billion Diamond Market Crash Leaves De Beers Reeling
So easy, why hasn't anyone done it already????
I mean, most space travel ideas haven't been done, even ones that are many decades old. Shit's expensive and risky, both as an investment and an activity.
For the same reason most other things involving space haven't been done yet. We as a species suck at prioritising stuff and allocating resources