BlueNile.com, Tiffany & Co., Zales Corp (Zales and Gordon's) and Signet Jewelers (Sterling, Kay and Jared) all have their own styles of jewelry. But obviously they are not the only suppliers of jewelry in the world. There are plenty of other jewelry suppliers that are also looking to compete against these "Big 4," and that's who you should be working with.
The selection of jewelry on your website should be something different than what the Big 4 sell. Your jewelry should represent what sets you apart from the Big 4 so you are not competing directly with them. This way, you can realistically charge competitive prices for your jewelry since now you are only competing with other online retailers.
One caveat is that you should not include jewelry styles on your website unless they are re-orderable. There is a considerable amount of time that should go into the product description for each item. If you spend a lot of time on a unique item, you need to make sure the profit margin is worth it.
The next time you are at a jewelry buying show, look for a variety of new styles from your vendors that could become your competitive staple against the Big 4 online. If you don't already know, ask your vendors if the styles are hot sellers and if they are intentional knock-offs of a popular style carried by Blue Nile, Tiffany, Zales or Signet.
Over time, your website will stand out as a portal that provides something different, rather than the most popular engagement rings.