Fluorescence is visible light, almost always blue, that some diamonds give off while ultraviolet light shines on them. It stops when the UV goes off.
In more detail
GIA reports that about 25 to 35% of the diamonds it sees fluoresce, more than 95% of them blue. Only about 10% of fluorescent diamonds are Medium or stronger, and fewer than 0.2% look hazy or oily. Most moissanite is inert under long-wave UV. So a blue glow points to diamond, but no glow proves nothing.
On a grading report
A line in additional grading: strength under long-wave UV (None, Faint, Medium, Strong, Very Strong) and color. On GIA reports, "None" covers everything from indiscernible to very faint. IGI records strength and color when present.
Related terms
- Color grade: The color grade is a letter from D to Z for the yellow or brown tint of a near-colorless diamond. D is colorless and Z is light yellow or brown.
- Moissanite: Moissanite is silicon carbide, one silicon atom for every carbon atom, in a hexagonal crystal. It is a diamond simulant, and every moissanite in jewelry is laboratory-grown.
- Fancy color diamond: A fancy color diamond is a colored diamond with enough color for a fancy grade. To GIA, any diamond outside the D-to-Z scale is a colored diamond.
- Diamond grading report: A diamond grading report is an independent lab's record of what a stone is and how it grades. It is true for the day the lab examined the stone.