Diamond is not a good investment from my experience. Even in the case of traveling to producing countries (and you must be a good expert in diamonds matter) and getting a good quality diamond and with the consequent certificate, the price will never be recovered.
The most accepted certificate is the HRD of Antwerp (the most apreciate), and in the US and the IGI in the most current but there are also less valued certificates such as GIA and EGL.
In the case of a good HRD certified diamond, the repurchase price will not exceed 65% of the
Rapaport wholesale price, which is the reference value in the wholesale market.
Obviously you will have paid this diamond 15/20% more expensive than the wholesale price.
And when you want to sell it, nobody will pay you more than 65/70% of the rapaport.
For example, a diamond with a maximum purity "IF", top color "D", with 1 CT can cost you € 23K in the market. The same diamond is valued according to Rapaport and GIA certified at ~ 16K. If you find someone (profetional) to buy it, they will offer you at most 13K. The diamond market is very closed and it is only for those who are part of the mafia that makes it up. If you are an expert in diamonds and get recognise a good stone when is still in the rough state, maybe you can do a business if above all you can certify your diamond and make sure that it is not a blood diamond.
DIAMOND IS FOREVER.... Because you never get to resold it