Most diamond dealers should be able to tell you if a diamond is certified or not.
A certified diamond has been inspected and and graded and registered according to GIA (Gemological Institute of America) standards. A diamond grading report is issued showing a comprehensive analysis of the diamond.
Kimberley Certified
A certified gemologist can determine whether your diamond is a natural diamond or a lab-created diamond.
The largest GIA certified black diamond available in a quick Internet search is a Fancy black, cushion cut, 7.21 carat diamond with a retail price of US$18,125. You may be able to shop longer and find a larger diamond, and the cost will be comperable to its individual characteristics.
Any diamond is 'good' when it can be certified as to its characteristics: cut, colour, carat weight and clarity. A certified gemologist can perform this work for you.
Take your diamond to a certified gemologist who can help you value the pieces.
Certified diamonds should come with a certificate describing their certification, which can be verified on the GIA (the organization that certifies most diamonds) website. The seller should be able to provide this information.
A genuine diamond can be natural or lab-created. A certified gemologist can help you identify the source of your diamond, and whether the diamond is indeed a diamond.
Every diamond is worth what someone will pay you for it. A certified gemologist can help you understand your diamond's cut, colour, clarity and carat weight: every diamond is unique in these ways. Then, you'll have a basis upon which to set a price for your diamond.
This is an unusual designation. One Australian jeweler claims to be a 'certified diamond seller'. In a brief search, it's the only instance of the designation to be found online. Directly, the phrase is 'certified diamond seller'.
A certified diamond is one that has been evaluated by a professional in whatever certification method is documented by the certificate. For example, the Gemological Institute of America -- GIA -- certifies diamonds by documenting the colour, clarity, cut and carat weight of the diamond. This establishes the individual characteristics of the gem. A non-certified diamond can only be evaluated according to the expertise of the person evaluating the diamond, and usually without certification. It's characteristics, therefore, are uncertain.
Every diamond is valued according to its cut, colour, clarity and carat weight. A certified gemologist can help you evaluate your diamond.