The qualities of good diamond has characteristics of 4cs : Color, Clarity, Carats and Cuts
Color: Color wise diamond is categorized in D to Z category. How much colorless the diamond is the more purity it has. 'D' category of diamond is comes under the purest form.
Clarity : Clarity is very clearly seen by the jewelers through magnifying glass.
Cut: Diamond quality is also very much judged by its cut.
Carat Weight: A simple diamond with a high amount of carat weight id more expensive than the equivalent number of carats in multiple stones.
If the diamonds has all these qualities it is good to pick. If you still have doubt, you can consult some professional in dealing field.