A non-certified diamond and a certified diamond might look exactly the same, and that is because they are. Certification of a diamond doesn't change it, and doesn't necessarily mean it is superior. A certified diamond has legal paperwork stating it is a certain type and quality of diamond. This guarantee offers peace of mind to the buyer, and may help with insurance and resale later. A non-certified diamond does not have this certificate of guarantee.
Certified diamonds have undergone a thorough inspection to establish their grade and quality. This is done by an independent company or entity to assure there is no bias, and that the seller of the diamond is not inflating the diamond's worth. Many diamond sellers only sell diamonds that have been certified, as it is the only way to guarantee the diamond is the caliber they are describing. It also helps determine a resale and insurance value for each diamond.
Non-certified diamonds might be exactly the same as certified diamonds, but they lack the written guarantee. This might actually make a non-certified diamond cheaper, as it will be more difficult to sell. On the other hand, you might be paying more than the diamond's worth, as there is nothing to prove its quality and grade except the word of the seller.
Diamond Grading Report
Diamond grading reports are recognized worldwide; therefore, they create a stable and fact-based price for each diamond based on a uniform system. The report is based on the unique characteristics of each diamond. The report identifies and documents what is referred to in the diamond industry as the "4 Cs": cut, color, clarity, carat (or weight). Finish, fluorescence and symmetry are also noted characteristics. Each report is numbered, and the diamond is given a corresponding number that becomes a type of identification. In some cases, the diamond is even laser inscribed with this identification number. A non-certified diamond does not have this grading report and therefore, when you buy or resell ii you don't necessarily know the specifics of the 4 Cs for that particular diamond.
The GIA and AGS
The Gemological Institute of America (GIA) and American Gem Society (AGS) are the independent agencies responsible for these certification diamond grading reports, but a non-certified diamond does not pass through the GIA or the AGS for certification. These agencies are recognized worldwide for their high standards in diamond certification and a non-certified diamond, therefore, may not be recognized by buyers only willing to buy certified diamonds. There are other regulatory agencies that provide certification, however a non-certified diamond has not been reviewed by any regulatory agency.
There is nothing inherently wrong, necessarily, with a non-certified diamond and if you like the shape, color and look of a diamond, there is no specific reason to have it certified, especially if you intend to be the only owner. However, certified diamonds are more easily insured, as certification provides a basis for the insurance price, but insuring a non-certified diamond can be difficult. Ironically, it is possible that your non-certified diamond may be worth more than you are able to insure it for because of its lack of certification. Also, certified diamonds are more easily bought and sold than non-certified diamonds.