The value of loose diamonds is much easier to find out than diamonds which have been set into jewelery. It is possible to send loose diamonds to a gemological laboratory such as GIA, for example, and have them graded properly.
A diamond already set into a ring, may have inclusions or flaws hidden by the prongs or claws which hold the diamond in place or the back may be sealed making it difficult to properly subject the diamond to laboratory analysis. To do a proper analysis the diamond would need to be removed from the setting in this case.
If you are looking for a diamond of a specific type and quality then it is far better to look for loose diamonds and then have them set into the jewelery you wish after the grading and value of them has been established.
Firstly establish what sort and type of diamonds you want. Are they for a ring? Or a broach or some other piece of jewelery? What size do you want. And what quality of clarify and color would you like? This will all depend on your budget of course. The marketing by De Beers has been successful in establishing that 2 months salary is ideal for the cost of a diamond engagement ring. However this is simply a marketing ploy and your person budget will decide what you can and cannot afford.
It is a good idea to be very familiar with the types of diamonds, the weights, color and clarity available and what each is worth. There is no substitute for some learning, due diligence and a little common sense.
Provided the above is kept in mind the value of loose diamonds will not be quite as mysterious as it may have been before and it will be quite possible to locate and buy some excellent loose diamonds.