Fluorescence
Some people seek diamonds that produce this unique effect, while others definitely avoid it. The visible effects of fluorescence grades of faint, inert, negligible, and medium, can be detected by a trained gemmologist. A fluorescence grade of strong or very strong can make a diamond with a near-colourless grade look even whiter yet in some instances give the diamond a slight hazy or oily appearance. Diamonds with a strong or very strong fluorescence are priced slightly lower than other diamonds