Many circumstances contribute to the beauty of a diamond. In previous GIA on Diamond Cut articles, we have discussed some factors that help to determine the appearance of a polished diamond. Most of these factors are optical properties of the diamond itself. These include: the laws of reflection and refraction, critical angle, dispersion, polarization, and the three-dimensional shape of a diamond (see, e.g., our Article Archive; we also discuss some historical and contemporary viewpoints about how cutting choices and diamond proportions affect a diamond's appearance). Although these principles are important in determining the behavior of light in diamonds, other factors come into play as we look at diamonds in the "real world." These include color, inclusions, and the state of the diamond surface (i.e., its polish). Perhaps the most important effect on a diamond's appearance, on a daily basis, is the cleanliness of its surface.

Cleanliness is the one appearance factor that can be easily controlled by the diamond's owner. For worse—and for better—diamonds act like "grease magnets." That is, diamond surfaces are attracted to most greases. Before we examine how this might be a problem for diamond owners, let's examine how diamond's affinity for grease can be helpful.

In 1896, an employee of De Beers discovered that diamonds were not affected by water and yet stuck to grease. Grease also was not affected by water. These two facts led a chief engineer at De Beers to develop the first grease table as a method of separating diamonds from rock fragments (figure 1).

Figure 1. This diamond worker is scraping the diamond-encrusted grease from the surface of a grease table during the final step of diamond recovery at a mine.

Early grease tables consisted of five steps covered in grease. The diamond and rock fragments would then be washed with water over the steps of the grease table. The water would push the rock over the edge of the table into a trough while the diamonds would stick to the grease on the table. Then this diamond-encrusted grease could be scraped off and heated to melt the grease away, leaving the diamonds behind unharmed. Eventually, the grease table was developed into the grease belt, which was able to keep this process moving at a continual rate. Although diamonds today are generally separated from rock fragments by X-ray sorting [Footnote 1], the fact that diamonds are attracted to grease has been very helpful in diamond mining.

Unfortunately, if diamonds are attracted to grease, then grease is attracted to diamonds. Just handling a diamond with your fingers provides enough oils from your skin (the type of "grease" that diamonds most often come in contact with) to alter the way your diamond looks (figure 2). And although the exposed crown

Figure 2. Notice the change in appearance in this clean diamond (left) when it has oils on its surface from being handled by fingers (right).

of a mounted diamond would seem to be the most susceptible to oils, the pavilion is more prone to retaining oils, and consequently usually has more oil on its surface. This is because the crown's surface has a greater chance of randomly being wiped clean by clothing, towels, or other fabrics. The pavilion collects oils from fingers—as well as cosmetics, lotions, residue from soaps, and other materials—and is less often in contact with cloth surfaces that might wipe it clean. In some diamond settings the pavilion area is mostly inaccessible; oil and dirt that reach this surface can remain there unnoticed until the ring is removed and cleaned (figure 3).

Figure 3. These images show fairly typical oils and buildup on the diamond of a ring. The images on the left show the "everyday" condition of the diamond before it was cleaned. The images on the right show the same diamond after cleaning.

Diamonds also collect oils on their girdles. There are three basic fashioning styles of a girdle: bruted, polished, or faceted. The majority of diamonds today are fashioned with either a bruted or a faceted girdle (figure 4).

Figure 4. These images show examples of a bruted girdle (left) and a faceted girdle (right).

The smooth surfaces of polished or faceted girdles will collect oils in the same way as other diamond facets, but bruted girdles consist of finely pitted surfaces (like very fine sandpaper) that catch oils much more easily.

Dirt and skin oils provide the most obvious cause of worsened diamond appearance. The oils stick to the diamond, then dirt sticks to the oils; and a dirty surface looks less transparent than a clean surface. But even without dirt, skin oil changes the appearance of a diamond.

Oils act as a coating, a thin separate layer of material or that light has to travel through to enter or exit the diamond. Because this oil has a different refractive index (RI) for any wavelength than either diamond or air, it changes the behavior of light rays as they pass through it. There are many kinds of oil, and each may have a slightly different RI, but the range of RI values for most oils is about 1.4-1.5 [Footnote 2]. Remember that the critical angle (that is, the angle that determines which rays of light will be reflected from or refracted through a surface) is determined by the RI values of the various surfaces through which the light rays pass. The greater the difference between the RI of each surface, the smaller the critical angle will be; the result of this will be that fewer rays are able to transmit through the surface, and more are reflected. Because diamond has such a high RI (2.417) compared to the RI of air (1.000), diamond's critical angle is relatively small (24.5 degrees). Since the RI of oil (1.4-1.5) is less than for diamond, the critical angle for a diamond with oil on its surface is larger. This has the effect of allowing more light to escape (transmit) at every surface interaction (figure 5).

Figure 5. Left: A light ray striking the inner surface of a diamond is reflected back into the diamond because it strikes outside of the critical angle. Right: Notice how the same light ray, striking from the same angle, is now able to escape (transmit) through the pavilion facet because the oil on the surface has changed the critical angle.

In other words, light that would have previously reflected back to later exit the crown toward the observer now has a higher chance of escaping through the pavilion facets.

Hopefully, the importance of keeping a diamond clean is now apparent. Although many jewelry stores offer free jewelry cleaning, keeping a diamond clean can also be done at home using commercially available jewelry-cleaning products or ammonia-based glass cleaners. A simple plan to keep your diamond jewelry always looking beautiful is to soak the diamond in the cleaning solution overnight, once or twice a week. In the morning, remove the diamond from the solution and brush with a soft toothbrush to remove any extra dirt. Take extra care to brush the back of the diamond (the pavilion) as this will be the area that has collected the most oil and dirt. Then rinse with water and wipe with a lint - and oil-free cloth. Using this method will keep your diamond looking as beautiful as the day you purchased it.

A few methods should never be used when cleaning diamonds, especially those set in jewelry. Most of these methods involve the use of chlorine (as in household bleach) or abrasives (such as household cleansers). Interestingly, toothpaste may contain both of these and so should not be used. Abrasives and chlorine erode jewelry alloys which may loosen prongs, or even dissolve the metal completely. Even mild soap should be avoided because it can leave a film on the surfaces of the diamond.

If it has been months, or years, since your diamond was last cleaned, it might need to be cleaned in an ultrasonic cleaner. This type of cleaner sends high-speed vibrations through the solution that the diamond is soaking in. These vibrations "shake" the jewelry to quickly dissolve the dirt and oil off the diamond surfaces. The prongs of the setting should always be checked before using this cleaning process, however, since diamonds may fall out of loose settings. In fact, it is generally a good idea to have all jewelry checked occasionally (every six months if worn often) to ensure that all prongs and settings are secure. Jewelers are often quite willing to provide this service (gratis) for customers.

In short, although there is much discussion today about light behavior and the "optimum" proportions for a diamond, many factors besides proportions can affect diamond appearance in the everyday world. One way to ensure that your diamond will look its best year after year is the periodic cleaning of dirt and oils from its surfaces.

We hope that you enjoyed this article, and invite any feedback or comments that you may have. You may contact us by email at DiamondCut@gia.edu.

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[Footnote 1]  These machines pass diamonds and rock fragments through an X-ray beam, which causes the diamonds to fluoresce strongly. Pressurized air is used to separate the detected diamonds from the rock fragments. [back]

[Footnote 2]  Recall that RI values for materials are given for the sodium D line, at 589.3 nm in the yellow range of the visible spectrum. [back]