Nobody wants to buy a diamond.
The price of these stones is artificially inflated by De Beers and a small set of other diamond extractors, by artificially creating scarcity.
For decades, they had the marketing firepower and patience to invest in advertisement in order to induce people to believe that a diamond is the gift to make in order to celebrate a wedding or other joyful occurrences. In reality, the whole market is a scam artificially inflated by the astute players who rig it.
If you buy the best diamond jewelry and try to sell it afterwards you will recover peanuts compared to the original purchase price, no matter what your dealer from the poshest neighborhood in your city had told you to the contrary: there is simply no appetite for this kind of inflated products, it is just a marketing exercise to extract profit from the willingness of people to spend money in anticipation of weddings or other joyful occurrences.