If you purchase a car, TV, or a set of bedroom furniture the value will decrease with age. Twenty years after their purchase you can't even give them away and toss them out. Their value is now zero.
The time has come to sell your diamond, you may try taking it back to the jewelry store where you purchased it, the diamond keeps it value -- right?
A diamonds value does not drop like a TV, or bedroom furniture. However, if you purchased your diamond at a retail store it will take many years for its resale value to increase to the price of purchase. A jewelry store has no interest in paying you a retail price for your second hand diamond, they don't have to... they buy wholesale.
A retailer or diamond buyer will only purchase the diamond if they can make money. The price they are willing to pay needs to be less than they can purchase the same diamond on the wholesale market or there is no advantage in buying a used diamond.
The wholesale value of your diamond has to rise a large amount before you can get what you paid for the diamond originally.