A certified diamond is a naturally mined rough diamond or a polished diamond (polished from rough) that has been graded by a professional gemologist or a gemological laboratory.
Available well known gemological laboratories:
GIA - Gemological Institute of America.
AGS - American Gem Society for Diamonds.
IGI - The Institute of Gemology.
EGL USA - Gemological Laboratory.
EGL International - European Gemological Laboratory.