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
Because of nuclear physics.
Diamonds are just carbon atoms arranged in a particular repeating structure (a lattice). So you can go from, say, graphite to diamond without touching the atoms themselves.
Gold, on the other hand is not about structure, but the atomic cores themselves, which contain exactly 79 protons. Going from one type of atom to another type requires some form of fission (breaking larger cores into smaller pieces) or fusion (smashing two smaller cores together). I don't think there are (viable) fission "recipes" that yield gold. And fusion requires insanely high temperatures or pressures even for smaller atoms (were talking center of the sun here).
I fused with your mom last night.
Sigh, I'll go get the chainsaw ...
Cut it out, dad.