If you are buying some loose diamonds for your own personal setting, the quality will be assured if the diamonds are accompanied by a GIA Diamond Report or a GIA Diamond Dossier.
However, for smaller stones which are not being graded by GIA, then you will have to depend on the integrity of the seller for what he represents to you on the quality. Your seller may without the least intention to misrepresent his products, however, most of the time the Sales Associate or Sales Rep may not be aware of the quality of diamonds his/her employer is selling. These are the traders and retailers who depend on the representation of the quality by their own up-line suppliers.
The Color and Clarity grade of a diamond may be difficult to determine for one without experience in grading diamonds and without the proper equipment. Even the cut of a diamond may be difficult to ascertain. Say, if you are buying a loose diamond without a GIA grading report, the best is for you to look for the Cut which is not cut to the proportions of an ‘ideal cut’ diamond, instead of looking for the excellent qualities in the loose diamond.
You might be surprised that the human eye is very sharp when it comes to seeing dis-proportionate cut diamonds and it can pick up asymmetrical inconsistencies with a 10X loupe.
The graphic representations below and some brief notes will put you in good footing when it comes to buying some loose diamonds without GIA Grading reports.