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One of the Four C's for evaluating diamonds
by Gary Dutton

Diamonds are found in all colors of the rainbow, from clear, colorless, transparent stones to ink black ones. Colors of high saturation such as red and green, which have no modifying secondary colors, are very rare indeed and command world record prices. Collectively, these colored diamonds are called "fancy colored" diamonds, and include the fancy colored yellow diamonds found at the far end of the color spectrum of the so-called "white or colorless" stones we will focus on here. A discussion of the variety and causes of the different colors, which occur in fancy colored diamonds, can be found elsewhere on this site.

The best color for a colorless diamond is, in fact, an absence of color. However these stones are relatively rare, the most frequently found diamonds (~80%) being of industrial quality. Those suitable for gem use (~20%) usually have some yellow tint, due to the presence of a small amount of contaminating nitrogen. As one would predict, a truly colorless stone will carry a premium price, and the larger that diamond, the greater the premium per carat. For every carat of polished gem quality diamond, about 250 tons of diamond-bearing earth (kimberlite) must be mined.

Diamonds are graded for color only as loose, unmounted stones in the inverted position (table down, pavilion up), and under very specific conditions of lighting and background, and also the distance and angle of the observer in relation to the stone. A color scale has been devised by the Gem Trade Lab. of the Gemological Institute of America (GIA) divided alphabetically from D (the absence of color) to Z (fancy yellow color), and is recognized world-wide. Each letter grade represents a small range of color, and not just a single color. Diamonds are color graded by being compared to a set of master stones. Another important and well-respected grading lab is the American Gem Society Labs., which uses a similar grading system, but a numerical scale from 0 to 10 (See figure below).

Because both GIA and AGS grading labs. are known to be strict and accurate in their color and clarity grading, Dutton's Diamonds sells diamonds with ONLY these grading reports (certificates, certs) and no others.

Colors D, E and F are essentially without color and differ more in transparency. Colors G, H, I and sometimes J, will usually show little or no color in the face-up position (as set in jewelry) for most diamond shapes. However, emerald cut diamonds, which have a large table and large, parallel, step-cut facets will more easily show color from G and above, especially in larger stones. Conversely, sometimes very well cut, round brilliant diamonds of ideal or super-ideal cut quality may show slightly less color than the grade given.

Effect of color on price: In general, as the amount of yellow in the stone increases, the value of a diamond decreases, that is until the fancy color grades are reached, and then the value goes up again. Roughly, for an internally flawless (IF) round brilliant diamond of 1-1.5ct., prices fall about 25% in going from D to E color, and then about 10% more for each additional grade (F=-35%, G=-45, H=-55%) until one gets to H color, where the difference decreases to about 5% less going from H to I color (I=-60%). So the first color decrease from D to E will provide you with by far the largest increment in saving money. Please keep in mind that these numbers are only rough estimates, and are not meant to be used as a pricing guide.

Dutton's Diamonds only infrequently offers diamonds of less than I color, and never sells diamonds of less than M color.

Factors Affecting Color

Other factors which can affect color include fluorescence, color enhancements such as high pressure, high temperature treatment (HPHT) and irradiation.

Fluorescence, produced by ultraviolet light from the sun, by black lighting or other long-wavelength UV source, occurs in an estimated 35% of gem grade diamonds. (Virtually all diamonds fluoresce when exposed to X-rays, and this forms the basis for their identification and collection at mining sites.) The UV light excites electrons in the diamond crystal, which then release this absorbed energy in the form of visible light, producing a blue, or sometimes other color, of faint to very strong intensity. Once the light source is removed however, the fluorescence is no longer observed. If, in rare situations, light emission continues for a period after the exciting light has been turned off, the phenomenon is called phosphorescence.

Blue fluorescence, if strong or very strong, may alter the perceived color of a diamond in a negative or positive way. For example, stones in the colorless/near colorless ranges (D-H) may appear milky or oily, detracting from their appearance. On the other hand, diamonds in the lower, more yellow color ranges (I and lower) may appear to have less yellow color due to the fluorescence, adding positively to their appearance. Thus, the trade will slightly discount prices of diamonds in the former category, while sometimes adding a very slight premium to those in the latter.

The presence and color (most frequently blue, but can be any color) of fluorescence and its intensity (none/inert, faint, medium, strong, very strong) are indicated on all GIA and AGS grading reports. However, these terms are considered by GIA to be only descriptive, and are not grading terms. (For an in depth study of blue fluorescence in diamonds, see: Gems and Gemology, Vol. 33, Winter, 1997, pp. 244-259.)

When shopping for a diamond, check to see if it has fluorescence. If it does make sure that it is BLUE and not some other color. The last thing you want in a colorless diamond is a potentially interfering fluorescing background color such as yellow, green or orange, which might greatly detract from the stone's appearance. If the diamond has blue fluorescence, it probably won't be a factor in the stone's appearance UNLESS the intensity is strong or greater, and even then only infrequently is it a problem. Nevertheless, make sure you verify that there is no effect of fluorescence under all conditions of lighting - from bright sunlight to all types of artificial lighting.

High Pressure/High Temperature (HPHT) treatment, is a process developed by General Electric whereby type IIa diamonds of low color (N-O) or even fancy brown color, can be converted to colorless/near colorless (D-H) forms by an annealing process involving high pressure and temperature. For details, see: Gems and Gemology, Vol. 36, Fall, 2000, pp. 192-215. There is some concern that these color-enhanced stones may come to market undetected, but new research has shown that they can be largely detected using expensive and sophisticated equipment. Similarly, the HPHT technique has also been applied to type Ia brown diamonds by several companies, yielding fancy yellowish green and greenish yellow colors. Pink and blue colors have also been produced by the same technique.

Irradiation of diamonds using a nuclear reactor or linear accelerator, sometimes in combination with heating, is used to produce a variety of different fancy colored stones.

Dutton's Diamonds does not sell, or deal in diamonds that are color enhanced in any way, so you can be assured that you will not receive an artificially enhanced stone from us!

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Gary Dutton, Ph.D., G.G.