No. Debeers only increased the price of diamonds by creating an artificial scarcity on the market shutting down rivals and not putting as many gems on the market.
Diamonds are plentiful in many areas, dotting the soil layers when they are found. And we can manufacture them, too. Gold is a better choice because we can't make it, and even that is a bad choice as an investment.
If you're concerned about currency collapse / devaluation, stock up on beans, rice, bandages, supplies you need in an emergency. Hold cash that will have value even if the banks limit your ability to transfer money, or pay off debt so that limits on cash transfers don't make you unable to pay your car payment.