I wouldn't choose any of these. Gold goes down and I predict that it will go even lower. Bitcoins are not stable and who knows what will be later. If we are talking in depositing in some kind of materials for some years I would prefer diamonds. I don't say that their value won't fall, but I'm sure that they won't fall as fast as gold and bitcoins.
The diamond scarcity is fictitious, it's a scam to keep the prices up. Bitcoin is the only supply that cannot be manipulated at will, im going to trust this for the future.