The best advice I think I could ever give anyone is when buying a diamond ring...NEVER, never, never buy it at a jewelry store. Instead, find a diamond broker in your town and buy it there. Jewelry stores typically sell crap to the masses. It's nearly always over priced. Walk into any jewelry store and look up. Track lighting is everyhwere. Track lighting makes everything sparkle. Also, most engagement rings are already in the setting. They do this to hide yellowing in poor stones. If you are going to make the investment, get a nice loose, high quality stone from a broker and have them set it. Go quality over quantity. Get something she can proudly pass on to her grandkids, or great grandkids someday. Don't get sucked into the bullshit jewelry stores dish out.
When I was a young man and was getting engaged for the first time, a friend turned me onto a broker in my town. His office was in a high rise building, with no advertising of any kind, yet he had been in business for over 30 years. I walked in and he gave me a diamond tutorial for about an hour. We walked into a room with track lights overhead. He took a loose stone and placed it in a setting and carefully handed it to me. "Look at this one", he said. "See how it shines?" And it did. Under the light it sparkled like crazy and the ring was gorgeous in the setting. "You like this one?" he asked. "Yes..its beautiful." I answered. He then carefully took it from me and dropped the diamond onto a blank piece of white copy paper. The diamond was nearly as yellow as a crayon, and it was cloudy. He turned off the track lights and it stopped sparkling too. "How about now?" he asked.
Any diamond broker worth his salt should be able to do the same for you. They can save you thousands. My ex wife got a smaller stone than many of her friends, but damnit she and I knew that her stone was a perfect, museum quality diamond. In fact she has since remarried, but plans to offer the stone someday to my sons bride should he every get married.