Fluorescence Grade Guide

Fluorescence is how a diamond glows when exposed to UV light, most commonly in a blue color. The diamond trade has historically discounted strongly fluorescent stones, especially at the top of the color scale, because strong fluorescence can occasionally give a D or E color stone a slightly milky or hazy appearance in sunlight. But fluorescence is not a flaw in itself, and for warmer-colored stones (I, J, K and lower) medium blue fluorescence can actually make the diamond look whiter in daylight by canceling out the warm body color.

The fluorescence discount is real and sits mostly at the top of the color scale. A D color stone with Strong fluorescence can trade 10 to 20 percent below its non-fluorescent equivalent, because the market is worried about the hazy-in-sunlight risk (which shows up in only a small percentage of strongly fluorescent D stones). At I color and below, the discount disappears or even reverses, because fluorescence is working in the buyer's favor visually. None and Faint fluorescence are always safe and carry no discount.

The simple rule: at D to H color, stick to None, Faint or Medium fluorescence; avoid Strong and Very Strong unless you inspect the stone in natural daylight first. At I color and below, Medium is often a bonus and worth considering for the color-boosting effect in daylight. Very Strong is rare and best avoided without an in-person inspection.