What is a certificate, do i need one if i want to sell my diamonds?

A certificate is a diamond grading report issued by an independent laboratory detailing the diamond's weight, dimensions, color, clarity and cut. The GIA (Gemological Institute of America) and EGL (European Gemological Laboratory), whose standards are the strictest in the diamond industry, are the undisputed international authorities on diamond grading and gem identification. The GIA issues a Diamond Grading Report or Diamond Dossier. The EGL calls their report a Diamond Certificate.