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
The breaktrough for asteroid mining isn't propulsion but how to mine and process ore in a vacuum.
Actually it's surviving the radiation outside the Van Allen belts.
Robots. Autonomous probes.
Available free to anyone who can hack the control codes!
Has anyone hacked the mining robots already in use on earth? Has anyone hacked any of NASA's probes?
How would you know if anyone did? If it happened on earth, the owner would say 'oh' and walk up to turn it off. You can't do that in deep space.