Cut: A well-cut diamond exhibits brilliance and sparkle.
Color: The less color (closer to colorless) the diamond has, the higher its quality.
Clarity: Diamonds with fewer internal or external flaws are of higher quality.
Carat Weight: Larger diamonds tend to be more valuable, but it's important to consider the other Cs as well.
Additionally, diamonds should come with a certificate from a reputable gemological laboratory, such as the Gemological Institute of America (GIA), verifying their authenticity and quality.