Loose diamonds are probably the best way to purchase diamonds.
You do not have to pay for a setting and it is easier for a valuer to check the value of a loose diamond than one in a setting.
Loose diamonds are also easier to buy, sell and transport around. Generally you do not get as much for selling diamonds as you paid for them. This is due to the fact that dealers on both sides want their costs paid and to make a profit. Never the less using diamonds to transport assets and value has been a practice for a considerable number of years.
One can purchase loose diamonds on the internet from reputable dealers but it is important that you get an independent valuation from an independent laboratory to ensure the diamond is as specified by the dealer. Particularly if you are buying diamonds sight unseen.
You should purchase a diamond at the top of your budget to get the best possible you can. Also buy with the reason in mind. For example if you are buying to have a stone set, what sort of setting and cut do you want? This web site contains all the information you need in the links on the right to determine the cut, clarity, color and carat weight.
These are the factors that determine how much a diamond is worth and what you will have to pay for it so you need to ensure that you understand the grading and clarity of the diamond, this plus the weight and cut determines the value and the price of the diamond.
When dealing with diamonds there is no substitute for doing due diligence and this is probably what will help you best of all when it comes to dealing in loose diamonds.