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
Ah, yes, the notoriously accessible and inexpensive field of launching rockets, landing said rocket on a giant sized bullet so a robot can go drill in it for weeks to dig up thousands of pounds of rock, lifting the robot that now weighs significantly more off of the giant bullet, and safely returning it to our larger, slightly slower bullet.