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
Gold mining is fairly expensive. According to this website, gold mining currently has a operating profit margin of 30%, while the average public company has an operating margin of 27%.
This lines up relatively well with this other dataset of US companies reporting 32% gross profit margins (go to the bottom of the page).
So it seems to me that gold mining isn't really that "special" compared other commodities.