First, think about your reasons for wanting to purchase the diamond online, as opposed to making a purchase from a local jewelry store. The most common reason is price. Due to low overhead costs, online jewelers and wholesalers are able to offer lower prices. However, you must be careful - sometimes a price that is too low is a sure indication of a scam.

One of the best things about purchasing online is the unlimited selection. When shopping offline, you are limited to the selection in the stores in your general area. Online, there are no limits. But again, you must use a great deal of care and consideration before handing your money over to someone that you cannot see and have never met!

Before shopping, learn as much as you can about diamonds - especially cut, color, clarity and carat weights. When you are knowledgeable about diamonds, it will be harder for a con artist to rip you off. Once you know more about diamonds, you will be ready to start shopping.

Take your time. Don't purchase the first diamond that you see that interests you.
Instead, look for similar diamonds for sale. Do some comparison shopping to find the lowest prices. Once you have found the lowest price, start doing your investigation.
You know about diamonds, you've found a diamond that you love, and you've found the lowest price - but you are still quite a ways away from actually purchasing that diamond!

Ask about the seller's credentials, such as professional jewelry associations that they belong to. View and print the seller's return, refund, and upgrade policies. Also inquire about additional services, such as settings and mountings, sizing, and free shipping. Do a search for customer reviews on this particular company around the Internet. Also check with the BBB Online to see if there have been any complaints.

Ask for a diamond grading report from an independent laboratory such as GIA, HRD, EGL or AGS. You should see this before making a purchase. Finally, use a reputable escrow service for high dollar diamonds - preferably one that will have the diamond appraised while it is in their possession. The seller sends the diamond to the escrow service, and you send the money to pay for the diamond to the escrow service. The escrow service has the diamond appraised, sends the diamond to you, and sends the money to the seller. This is the surest way to protect yourself...again, make sure that you use a reputable escrow service!