Educate yourself about diamonds and jewelry in general and diamond carat weight, diamond quality, and gold karat weight specifically. Also find out what the differences and similarities between diamond reports are and which organization’s reports you would prefer to have your diamond certified by.
Some valuable resources are:
http://howtobuyadiamond.gia.edu/Tour_FrameSet.htm
http://jewelry.about.com/od/diamonds/u/jewelry_advice.htm
Start your search by entering as specific as possible of a “search term” in one of the major search engines. For example if you are looking for a pave set diamond right hand ring than enter “pave set diamond right hand ring” in the search box. This will help you find sites that are more specific to your search rather than having to go through hundreds of sites that are irrelevant to your search.
Browse through many sites before making a purchase. Make sure you are comparing apples to apples when evaluating different products. An average online shopper for diamond jewelry pieces takes between 10 -15 days before making a purchase.
Some suggested online jewelry sites:
http://bluenile.com/
http://jamesallen.com/
http://clevereve.com/
http://zales.com/
http://novori.com/
Make a list of the items and the URLs of the product pages of the items you liked.
Educate yourself about the site you are considering purchasing your piece of jewelry from.
Read the fine print and the warranty policy carefully. Most reputable online jewelers offer a 30 day return policy in addition to a limited manufacturing warranty policy. It is also more common than not for a high end diamond jewelry sites to offer free insured shipping on purchases over a certain dollar amount.
Inquire about the site that you wish to purchase your product from on independent, third party sites and forums. A good point to start your search is the BBB. Also check to see if the site has a valid SSL certificate (preferable a SSL 2 certificate which certifies the business entity that is behind the operation of the site). Usually this can easily be checked by looking for the buttons of these providers on the bottom of the pages of the site you are interested to purchase from.

Read customers reviews about the site that are places on a third party – independently operated site.
Some examples are:
http://ivouch.com/
http://trustlink.org/
http://bizrate.com/ratings_guide/guide.html
Make sure the site is operating from within the United States and has a physical location. Their mailing address must be present on their site – usually in the terms of use or privacy policy section of the site.
Call their toll free number to place an order rather than going through the online shopping cart. If the site does not provide a toll free number simply forget about purchasing anything valuable from that site. When placing your order ask for discounts and promotional offers. You might be able to save a few bucks just for asking.
Have a good time shopping for your diamond jewelry piece and remember that the process of purchasing a diamond jewelry item online is supposed to be an easy process. If you are having a lot of trouble just calls the toll free number of the site. There usually are knowledgeable representatives behind the line that can eliminate most of the anxiety and burden related to purchasing a valuable item online.