Diamonds are not a great investment. Russia and De Beers stockpile them and keep their price artificially high. If for some reason, they decided to sell them off, the price of diamonds would crash...
They restrict the supply which keeps the price UP!
They will NOT OVER SELL them since that will affect their profit.
I can think of NO reason why they would bring down their VALUE.
Other options are investing overseas or in a domestic based start up. The only problem with those is if you make money off them, you'll be taxed accordingly. That's another reason why diamonds and precious stones are good because they sit there and don't call attention to themselves after they are purchased and they are portable. Any country in the world will have someone that will purchase them in cash - that is how it is done ... .