How to read a GIA report, line by line

Carat weight is the number everyone jumps to, and the least interesting one on the page. Two stones of identical weight can differ enormously in how much light they return. What you want to understand first is the cut grade, because it is the only parameter that describes human skill rather than geological luck.

Color comes next, and here the grading letters hide a simple truth: the difference between adjacent grades is often invisible once a stone is set. Compare two stones loose, side by side, and the gap appears. In a finished ring under mixed light, only the largest steps are worth paying for.

Fluorescence is the most misunderstood line on the report. In the vast majority of stones it has no visible effect at all. It can soften the color of a warmer diamond in daylight, which is sometimes a benefit, and the market discount it carries is frequently larger than the effect justifies.

Finally, read the plot. The map of inclusions tells you whether a stone is clean because it is pure or clean because the inclusions were hidden by the cutting. Both are legitimate. Only one of them predicts how the stone will behave if it is ever re polished, and that is worth knowing before you buy.