The 4 C's of diamonds are a standardized system used to evaluate and grade diamonds based on their characteristics. These four criteria are:

Cut: The cut of a diamond refers to its proportions, symmetry, and polish, as well as how well it reflects and refracts light. A well-cut diamond will exhibit brilliance, fire, and scintillation. The cut grade is determined by factors such as the diamond's proportions, angles, and the precision of its facets.

Color: Diamond color indicates the presence or absence of color in a diamond. GIA grades diamond color from D (colorless) to Z (light yellow or brown). Less color signifies rarity and higher value, except for fancy colored diamonds, which may be highly sought after.

Clarity: Clarity measures imperfections (inclusions) and blemishes in a diamond. GIA scale ranges from Flawless (no visible imperfections at 10x magnification) to Included (visible to naked eye). Grade based on size, nature, position, and number of flaws.

Carat Weight: Carat weight is a measure of a diamond's weight, with one carat equal to 200 milligrams. Larger diamonds are usually more valuable than smaller ones, but two diamonds of equal carat weight can differ in value based on cut, color, and clarity.

These four criteria collectively determine the quality, beauty, and value of a diamond. It's essential to consider all four factors when selecting a diamond to ensure you're getting the best combination of characteristics for your preferences and budget.