You can consider a diamond certificate issued by an independent gemmological laboratory reliable because:
These laboratories don’t sell diamonds and have no connection to diamond mines, dealers or retailers, so they have no conferred interest in the grade they give a diamond.
These labs depend on repeat business so they have to maintain a reputation for accuracy and integrity.
They use diamond graders and gemmologists who undergo intensive training and have passed tests assessing their ability to correctly grade diamonds.
The grade is based on a scientific reading of the diamond’s characteristics using specialized equipment designed just for grading diamonds.
Several graders evaluate an individual diamond separately. Because no two diamonds are exactly alike, the graders reach a consensus to make sure the grade they assign is accurate, consistent and reliable.
Each diamond certificate is marked with a unique number that matches the exact diamond it describes.
Some grading labs offer optional services, including microscopically inscribing the diamond certificate number on the edge (or “girdle”) of the diamond as a security feature.