Try to refrain from the term “certification” when referring to lab reports. A certification implies a guarantee and a lab report does no such thing.
A lab report can be an inavaluable resource to improve the salabilty of a diamond (but not necessarily the value). Value can be clearly added when the specifications of the diamond are favorable (especially in the ideal range). Some gem labs are better than others but even the most reputable are apt to make mistakes in there grading. GIA (Gemological Institute of America) is probably the best known in the U.S., but there are others that do lab reports equally as well. AGS (American Gem Society) and PGS (Professional Gem Sciences) are both very proficient in diamond and precious gem lab reports. There are a host of others including EGL, IGI, AGA, HRD and more. The important thing to remember in any lab report is not to take the accuracy for granted until you, the expert, have verified it.