You take your diamond ring in to sell to a reputable jeweler and they offer you $900.  What a dissapointment!  You got a great buy and  bought the ring for $2500 and the jeweler gave you 50% off the retail price of $5000.  They even gave you an appraisal for $5000.  Was I ripped off originally?  How come I get less than 1/5 of the appraised price.

Right now, you should focus on getting the most from the ring you don't want anymore.  In most cases, jewelry stores don't "rip you off."  They are a legitimate business trying to sell for a fair profit to sustain their company.
Appraisals at a retail store are not impartial.  They are given by the store to allow you to insure your diamond ring against risks of loss, the diamond falling out, the ring stolen or the diamond getting chipped.  When you bought the ring, you didn't buy it with the intention of selling it, so don't blame the jeweler that sold it to you.  The company that eventually buys it from you is entitled to a profit.  They may be able to buy the ring again from a diamond dealer for $1250, but they may get consideration from their normal sources such as easy payment terms, like 90 days to pay for it.  They also want to be important to their vendors so that they have a better business relationship for the years to come.  They may need a favor from that vendor next year.  They may hit upon financial problems of their own and need to extend their payments for product they bought.

Your job is to find the best offer for your ring.  They money you spent several years ago is gone.  Make the most of what you have and get some more bids and take the best one.