Safe bets are retailers associated with Jewelers of America or the American Gem Society. For diamonds, first decide what shape, carat weight, and color grade you want. Next, compare the cut and clarity of different diamonds that fit the bill. If the jeweler doesn't stock loose stones, move on. If the piece includes two metals, there should be a quality mark for each.
Most retailers dramatically inflate prices.
Never pay the sticker price unless you've shopped around and you know it's already a fair price. If no report is included, make the sale contingent upon an independent appraiser's opinion. Last but not least, plot the diamond's inclusions and blemishes on a drawing of the stone.