I have bad signal so I can’t find the sources for you, but if you look up something like “diamonds artificial supply limiting” you should find it.
Basically the diamond industry has a stranglehold on the supply of natural diamonds (which are also notoriously mined under horrid conditions - i.e. “blood diamonds”) and they artifitially alter the supply/demand ratio by limiting what goes onto market to jack up the price. Naturally, diamonds are actually a very common gemstone, and they can be easily created in a lab. They’re just hyper-compressed carbon, after all, and carbon is abundant on earth. Rubys, sapphires, and the like are much naturally rarer, but don’t fetch as high a price because of the market manipulation around diamonds.