Carat
This one changes what you see. It is worth spending on.
Carat is weight, not size: 1 carat = 0.2 grams. Two stones of the same carat can look different sizes depending on shape and cut. Elongated shapes (oval, pear, marquise) look larger face-up than rounds of the same weight.
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Certificate · Clarity · Color · Cut · Eye-Clean · Fluorescence · Lab-Grown Diamond · Ratio · Table and Depth