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
What about those things require any g?
Asteroid miners are going to have more in common with spiders than anything else; find a tasty roid, wrap it up in cling wrap and set off bombs inside of it until it's a bag of gravel, then get the whole mess spinning and just let go of the stuff you don't want to keep. Hell when you're done you can re-harvest the angular momentum to propel you over to the next roid and start the process over again.
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