Acting as a prism, a diamond can divide light into a spectrum of colors and reflect this light as colorful flashes called fire. Just as when looking through colored glass, color in a diamond will act as a filter, and will diminish the spectrum of color emitted. The less color in a diamond, the more colorful the fire, and the better the color grade.
	D: Absolutely colorless. The highest color grade, which is extremely rare. E: Colorless. Only minute traces of color can be detected by an expert gemologist. A rare diamond. F: Colorless. Slight color detected by an expert gemologist, but still considered a "colorless" grade. A high-quality diamond. G-H: Near-colorless. Color noticeable when compared to diamonds of better grades, but these grades offer excellent value. I-J: Near-colorless. Color slightly detectable. An excellent value. K-M: N-Z:
The color in diamonds graded K-Z detracts from the beauty of a diamond. It's especially noticeable set in platinum or white gold.
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 only be detected by a trained gemologist. A fluorescence grade of strong or very strong can make a diamond with a near-colorless 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.
For the purist, look for a colorless diamond with a grade of D-F and a fluorescence rating of faint, inert, none, or negligible. For an excellent value in a diamond with no noticeable color to the unaided eye, look for a near-colorless grade of G-I, and a fluorescence grade of medium or strong blue. Or, if you'd rather not compromise on color but would like to stay on budget, choose a diamond with a good cut, SI1–SI2 clarity, and consider going with a strong fluorescence. It will still be beautiful to the unaided eye and you may prefer the unique effect of a strong fluorescence.