I just joined Stack Exchange and saw this very interesting question. I might be late with the answer but it is worth giving feedback. Please allow me to elaborate:
First of all, I have been in the jewelry manufacturing business for over 20 years. I was involved in the creation of Mondera.com (now closed as jewelry e-tailer), Gemkey.com (B2B jewelry/diamonds/gemstones trade site ) and Fameo.co.uk (selling jewelry online to the UK Market, now also closed).
In all these ventures, I was the co-founder and CTO but in Fameo.co.uk I was the owner. It was supposed to be the front end of our manufacturing facility based in Bangkok, Thailand. Despite it being closed for reason I won't discuss here as they are out of topic, we still serve the clients who need repairs done, adjustments etc.
To answer your question straight to the point:
- You need to make sure the site is operated by legitimate owners who KNOW the business: It is very easy for anyone today to come up with a site that sells jewelry online but this does not mean that the people behind it understand jewelry. It is preferable that you purchase jewelry from websites that are run by people who actually do manufacture them or at least have been in the business for long time and understand their product and customer needs.
- What you see is not necessarily what you get: Many sites out there retouch their products extensively and the retouched items can be far away from reality. In my idea this deceiving the client. Other sites display computer generated photos of their product and I do recommend you to absolutely AVOID such sites because there is NO WAY that you will get the product you see on screen. Jewelry making is far from being a perfect product like such CAD generated designs.
- You cannot determine 100% quality: You cannot know for sure of the quality of the item by looking at the photo but if the photo has not been retouched beyond norms, then you can get a very good idea of the product quality. At any rate, if you are buying from a company with reputation and who preferably have their own manufacturing facilities, check their history and what they produced and for which market. As technical questions (if you can) and see the answers (chat live if possible so you know the person knows the business and did not prepare for the answer).
From my humble experience, most of the products out there are mass produced and of low quality, few sites provide high end jewelry that you can satisfy your desires. In the end, what matters is trust. In our business trust is above all and if your online jewelry store can establish that trust and make you confirtable and assure you of your purchase then I am sure you will get back for more.
If you need to know more about my credentials to you know what I am saying is true, visit my G+ community https://plus.google.com/u/0/communities/100689153072781340162 to know more. I can help you making your purchase experience a nice one and I won't charge you for my advice :-)