![]() The faceted stone can be positioned in any orientation as long as the light transmitted through it is sufficient to reach the instrument’s detector. Since diamond is optically isotropic, the same spectrum can be recorded in any direction through the sample. Gemologists often refer to this set of dark bands as the “Cape” absorption lines characteristic of type Ia yellow diamonds.Īt GIA’s laboratory, visible spectra of diamonds are recorded with a spectrophotometer. In the case of this yellow diamond, superimposed on this increasing absorption are several sharp absorption bands or peaks on the spectrum trace between 415 and 478 nm due to the N3 optical center. Below 425 nm, the height of the spectrum trace increases. Greater absorption is represented by the higher portions of the spectrum trace, and greater transmission by the lower portions. Recording the spectrum with a spectrophotometer produces a graph of wavelength (horizontal scale) versus absorption (vertical scale). In the spectroscope, this selective absorption is seen as a darkening toward the blue end of the spectrum (below 425 nm). In the case of a yellow diamond, the blue portions of the incident white light are absorbed by the diamond, while the remaining portions in combination are transmitted to the eye and interpreted by our vision system as a yellow color. In most cases, transparent gemstones owe their color to selective absorption of light. Visible absorption spectroscopy is the analytical tool for understanding most causes of diamond coloration.įigure 1. When they exit the diamond, these transmitted wavelengths in combination can create the sensation of color in the human vision system (figure 1). Some of the light energies (i.e., wavelengths) are absorbed by the defects, while the unabsorbed wavelengths are transmitted. (In these instances, the lattice defects are often referred to as optical defects.) When white light strikes a polished diamond, some of the light is reflected, while the rest enters the diamond where it is refracted and dispersed based on wavelength. As discussed in our 2013 article, most diamonds also contain lattice defects which, in sufficient concentrations, can produce selective absorption of incident light. There are, however, intrinsic absorption features in the spectra of all actual diamonds. GIA’s laboratory staff, which has the opportunity to examine a large number of colored diamonds, faces this identification challenge on a daily basis.Ī theoretically pure and defect-free diamond would be completely colorless, and a unique attribute would be its transparency (or lack of light absorption) across a wide portion of the electromagnetic spectrum from the ultraviolet through the visible and into the infrared regions. It also discussed how spectroscopy techniques, used to detect absorption and/or emission bands caused by those optical defects, play a leading role in making this important determination. The brief article that accompanied the chart discussed the ongoing challenge presented by the identification of natural, treated, and synthetic diamonds. ![]() The chart presented some basic information on those defects, including the ones responsible for the colors and ultraviolet fluorescence reactions of most diamonds. In 2013, the authors published a simple chart listing the major optical defects that can occur at the atomic lattice level in diamond (Shigley and Breeding, 2013). ![]() The chart shows how similar diamond colors can result from different light absorption patterns. A chart has been prepared to illustrate the visible spectra of various colored diamonds as recorded at low (liquid-nitrogen) temperatures with a spectrophotometer. ![]() Yet the broader regions of absorption, which can be difficult to observe with the spectroscope, often have a greater influence on a diamond’s color. The spectroscope allows a gemologist to observe some of the more intense and narrower absorptions in the visible spectrum of diamond as dark bands at specific wavelengths. The unabsorbed portion of this light is transmitted through the diamond and is interpreted by the human vision system as the perceived color. Comprehensive CAD/CAM For Jewelry Certificateĭiamond color is usually the result of selective absorption of incident white light. ![]()
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