Learn as much as you can about diamonds. Always trust your eye. Shop around and compare 4. Compare the actual price you are paying, not the percent of discount.
Large discounts indicate higher mark-ups.
Compare quality and price together for value. Diamond prices vary drastically with quality differences. A cheap price is not always a bargain. Ask to see the diamond in a microscope or Dia View. A jewelers loupe is difficult to use if you are not experienced.