What is Fluorescence in a Diamond?   Gem-quality diamonds that emit light when exposed to long-wave ultraviolet light. B What are the Gradations of Fluorescence set by the Gemological Institute of America?   N: No Fluorescence   F/FB: Faint/Faint Blue Fluorescence   SL: Slight Fluorescence   S: Strong Fluorescence   EF: Extreme Fluorescence Why is Fluorescence Bad for a Fine-Quality Diamond?   Fluorescent diamonds, especially the extreme rare cases known as overblues, give a diamond a visible haziness that undervalues them on the market.   They appear almost cloudy in light with strong ultraviolet content, which compromises their clarity and decreases their appeal. Why is Fluorescence Good for Diamonds of Lesser Worth?   Strongly fluorescent diamonds with yellowish body color have the appearance of better, whiter color in sunlight, a source of ultraviolet light.   These lower-color diamonds with powerful fluorescence often command a premium. Fluorescence Put to the Test by the GIA   Fluorescence in diamonds is considered rare. 65% of diamonds have none.   Strongly blue fluorescent diamonds possessed better color grade in the face-up position.   No fluorescent effect was observed in the table-down position, which is how labs color grade diamonds.   Color fluorescence was more apparent in diamonds grades I through K.   Strongly or very strongly fluorescent diamonds were considered to have better color appearance than less fluorescent stones.   Blue fluorescence had little effect on transparency.   Overblues are rare, so they were not included in the GIA study.   The GIA conclusion: fluorescent diamonds are stigmatized needlessly, which puts a negative impact on their salability.   Fun factoid: The famous Hope Diamond fluoresces an extremely unusual red-an occurrence in blue diamonds-and this has influenced the legend of its curse.