by MissyBeaucoup » Sep 1, 2018 OK, so this is a fun little game to play with your colored gemstones. All you need is a camera and a loupe with a UV light. (I got mine inexpensively on Etsy from FDJtools in Florida.) What is fun is that stones colored with chromium will fluoresce red, but if they have very much iron (like blue sapphire), the iron will block the fluorescence. Maybe somebody can explain it better than me! Anyway, the Mahenge spinels fluoresce really strongly and so does ruby, like in the center of my avatar ring. The demantoid garnets don't fluoresce. Feel free to post your own favorites too!!