0.40 ct GIA Diamond Solitaire, F SI1

0.40 carats, GIA certified F colour and SI1 clarity, set in 18K white gold.

Colour and clarity are the two lines everyone reads on a GIA report. They are not the two that decide how a diamond looks. Here is what else is on the page, and why it matters more than most buyers are told.

Cut grade. Only round brilliants receive one, and it is the single strongest predictor of brightness. A poorly proportioned stone leaks light through the pavilion and looks dull at any colour grade. Read this line first.

Polish and symmetry. Graded separately from cut. Symmetry describes whether the facets meet where they should; poor symmetry scatters the pattern of light return and is visible as a slightly restless, unresolved sparkle.

Fluorescence. Roughly a third of diamonds glow blue under ultraviolet. In faint to medium amounts it is neutral and occasionally flattering on a warmer stone; strong fluorescence can give a milky look in daylight, and it moves the price.

F and SI1 tell you this stone is colourless and eye-clean. The rest of the report tells you how it behaves.

Same weight and grades, narrow shank: 0.40 ct F SI1.