As someone who use to sell diamonds and jewels, most of you will lose 50-70% selling your diamonds and jewels unless you really know what you are doing and know how to grade diamonds, distinguish fakes and can find a retail end buyer. And few people are seeking to buy just an individual diamond, they usually want it in a piece of jewelry. I eventually ran into this inventory problem that when I bought $200,000 worth of diamonds (trade not retail price) it took me a really long time to sell them off for this reason. The market has also only gotten worse because of bad PR for diamonds, and movies like blood diamond, there is international regulation on source origin of diamonds to try to stop blood diamonds from crossing borders, a decline in marriage and expensive weddings where diamonds where mainly bought. Also diamonds are not as rare as people thought they were, they are common in Africa, hence the diamond companies have to create chaos, wars in places like Liberia, Congo, Sierra Leone etc. and lots of laws in Africa and world wide to try to prevent the market from being flooded. Hence these blood diamond laws are really about just stopping the Africans from selling their own diamonds and flooding the market and undercutting established players/cartel members. Funny how there is blood diamonds but no blood oil, blood rubber, blood, coltan, blood copper, blood gold, blood tomatos, blood cacao, or blood chocholate or blood coffee.
It is widely believed that De Beers, and several other major diamond cartel players bought up billions of dollars of easy access diamonds from Africa in the 60s-00s and store them in vaults in Europe and they can flood the market any time.
Every diamond dealer thinks that a person selling their diamond is desperate, they also have a large inventory that they are struggling to sell, so do you think they are going to offer you market rates knowing that 90% of the people who do sell diamond to them are women trading in old rings and don't know its value because they didn't pay for it? They'd rather rip you off or make no deal than pay you fairly.
I don't know the answer to the question, but diamonds would be towards the bottom of my list of assets, I wouldn't be keeping a significant amount.