It can now be easily and cheaply mass produced (e.g. synthetic diamond).
The only difference between synthetic diamonds and "natural" diamond is that the latter may possess some kind of defects.
So people are pretty much paying a premium on 3D covalently bonded compressed carbon not for the Diamond itself but for those defects (which can also be forged in a proper high-end lab).
The value of diamond has more to do with marketing which generates demand than anything else.
At least cryptocurrency have actual real-world use. BTC -- as a store of value, ETH/NEO/Stellar ICO platforms, Stellarport/BitShares (Decentralized Exchange and Banking), VeChain/NucleusVision logistics tracking and retail services enhancement, and so on..
What can jewellery grade diamond (e.g. not the cheap industrially useful synthetic ones, but those overpriced ones) do other than the hype around it from intensive marketing and media portrayal?
I predict the price of diamond will go to $100/kg
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lol of course I am kidding with the last statement and the title. But I am just drawing parallels between other commodities (using diamond as just one of many possible examples) to illustrate a point -- and to counter the massive FUDs being spread by jealous tradition financial institutes (many of them have even crashed the world economy in 2008 w/o any consequences, bailed out by tax payer's money, and the top executives used that to pay themselves huge bonuses) -- these traditional financial institutes will certainly be disrupted by the emergence and rise of cryptos, and they are now panicking and fighting back their "control" against an entity that they could not as easily manipulate.
You can't actually produce nature diamonds, the produced diamond won't be as strong as the real one, and it can be easily determined as a fake diamond.
More then that, I think that relying on gold and diamonds is not good for the long run, as the techonology is just getting better and we can't really know if gold/diamond will still be elements that it's impossible to fake them.
On the other hand we can be sure that bitcoin won't be faked at the long run as it's nature requires everybody to agree for changes.