The 4C’s are universally accepted criteria used in diamond evaluation. Diamond certification is often considered the 5th C as it provides the stamp of authenticity you'll need to insure, or to sell your diamond.
The 4C’s are: Carat - Weight of the diamond Cut - Quality of the proportioning, polish, and symmetry Color - Diamonds with no color like are very rare and expensive Clarity - How free a diamond is from natural flaws
See a detailed diamond certificate.