Have reasonable expectations. Sentimental value doesn't count here. Jewelry is just a commodity, and price is determined by the market value. Some items hold their value better than others. Co. But you may get less than half the original purchase price for a diamond because there is such a markup at the retail level.
Some people expect to get more than they paid originally.
That won't happen unless it's a true antique. If you still have the original receipt, box, warranty, accessories or anything else that goes with a piece, bring it along. It helps prove the item's authenticity. Don't bother bringing an appraisal, though, since these are often inflated for insurance purposes and don't reflect the jewel's true value. Estate jewelers and pawnbrokers that specialize in jewelry do their own appraisals.