Diamonds come naturally in almost every color of the rainbow.  But, most people are concerned with diamonds in the white range.  Colorless diamonds are more valuable, because they are rarer and it is the lack of color, or whiteness in a diamond that allows the light to pass effortlessly through the stone and disperse that beauty back to the observer.  The color grading scale established by the G.I.A. (Gemological Institute of America) varies from D (totally colorless) to Z (light yellow).  D through F are virtually colorless.  G, H and I diamonds appear colorless when mounted.  J, K and L diamonds look very nearly colorless, but you can see some color in larger stones.   Beyond M, most people can see color pretty easily.  The color of the metal in a mounting can either mask or enhance the diamond color.  Yellow gold makes slightly yellow or brown diamonds appear more colorless.   White mounting  (gold or platinum) makes the color more perceptible.

Fluorescence is not formally a color grading term.   Many diamonds glow when exposed to light which contains relatively high amounts of ultraviolet.  It is due to natural interaction of light's energy and the atoms in the diamond.  Some diamonds (about 10%) fluoresce strongly enough so as to be somewhat   noticeable in regular (incandescent) light.  Generally, for very light yellow color diamonds, fluorescence is considered to be beneficial since it makes the diamond appear whiter.

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