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
Barbed harpoons were the approach with the comet. Putting a band around the whole thing could also work for a small body. If you're going for gold, the asteroid is also going to be paramagnetic if I'm not mistaken, so you can just electromagnet on.
It's slightly more problematic than on Earth, but I'm basically going to need some citations if I'm ever going to believe it's sticking to the chosen landing site that's the hardest part. A lot of proposals sidestep on-site processing, even, and rely on delivering a whole small body to Earth.
Maybe not to Earth (unless you wanna crash economies), but a LaGrange point would be suitable – as well as a place for something spinning so the hairless monkeys could have a semblance of gravity.
If you have human miners, anyway.