Diamond carat
For many people, size is the first thing they ask about. Carat is actually a measure of weight, not size, one carat is 0.2 grams, and the relationship between weight, visible size and price holds a few honest surprises.
Weight is not spread
Two one-carat diamonds can look different sizes face-up. A well-cut stone carries its weight in the right places; a deep-cut stone hides weight underneath where no one sees it. This is another reason cut comes first, it literally changes how big your diamond looks.
The threshold trick
Prices step up sharply at the popular weights, 0.50ct, 0.75ct, 1.00ct, 1.50ct, 2.00ct. A 0.90ct diamond can cost meaningfully less than a 1.00ct that looks virtually identical on the hand. Buying just under a threshold is one of the most reliable ways to get more diamond for the budget.
Proportion beats size
The right carat weight depends on the hand and the setting it will live on. Practical and proportional beats big and impractical, a stone you love wearing every day is the goal, not a number.