: Measures internal flaws (inclusions). Look for "eye-clean" diamonds where imperfections aren't visible without a jeweler's loupe.
: This is the most critical factor for a diamond's sparkle. An "Excellent" or "Very Good" cut can make a diamond appear brighter and even larger than its actual carat weight. tips for buying an engagement ring
: This refers to weight, not visual size. Buy "shy" weights (e.g., 0.90ct instead of 1.00ct) to save up to 20% on price with almost no visible difference in size. 10 Tips for Buying a Diamond Engagement Ring - GIA 4Cs : Measures internal flaws (inclusions)