A reaction to UV light. Fluorescence is graded from None to Very Strong. A fluorescence grade of Very Strong can make a diamond with a near-colorless grade look even whiter and could, in some very rare instances, give the diamond a slight hazy or oily appearance. Diamonds with a strong or very strong fluorescence are a better value for the end customer because the market prices them slightly lower than diamonds with faint or negligible fluorescence.
Grading report, grading certificate