If you are looking for a quality jewelry store in Miraflores, look no further than "Migue" on Ave. La Paz. In fact, there are two of them, one across the street from another. They have a huge selection, prices are reasonable, and the owners are pleasant and honest. I have lived through 15 Valentines, birthdays and Christmas in Peru, and since I live on La Paz Avenue, I have had the chance to enter most of the jewelry shops there at least once.
Really... don`t waste your time with most of those places. Limited selection and doubtful quality. Go straight to Migue. You won`t regret it. I believe they are located on the 3rd block of La Paz (Between Cantuarias and Esperanza)