You will know exactly what you are buying. Every important factor you need to know about the diamond is described in the certificate. The seller knows exactly what he/she is selling to you so as to avoid any misunderstandings in the future. If a diamond comes with a GIA certificate, you do not have to worry about the possibility that you might be purchasing a fracture-filled diamond. These are diamonds that have surface cracks that can be artificially filled to appear as much better diamonds than they actually are. These are in essence, very low quality diamonds that look great to naked eyes. GIA does not certificate these diamonds.