A Guide to Diamonds with the 480 nm Absorption Band

ABSTRACT

In the gemological research community, diamonds with a broad absorption band centered at roughly 480 nm in the visible spectrum are referred to as “480 nm band diamonds.” Despite the 480 nm absorption band being one of the few causes of yellow bodycolor, diamonds with this feature are relatively scarce and not as widely recognized as those colored by the N3 defect, single substitutional nitrogen defect, or H3 defect. Yet 480 nm band diamonds deserve more attention because they account for the vast majority of rare diamonds with unmodified orange hues, as well as all chameleon diamonds—diamonds with a reversible color-change property. However, the global abundance and distribution of 480 nm band diamonds are currently unknown. To date, only mines in Russia and Canada are known to produce these diamonds, but other sources likely exist. Understanding their gemological and spectroscopic characteristics will help the mining industry and the gem trade identify these obscure treasures. This article summarizes the features of 480 nm band diamonds for rapid identification or advanced gemological testing.

Though natural diamond is composed of carbon, atomic impurities and vacancies commonly occur in the diamond crystal structure, attributed to the incorporation of extrinsic elements and strain induced during diamond growth. Post-growth irradiation and plastic deformation can also produce vacancies in the diamond structure (Van Enckevort and Visser, 1990; Fisher, 2009). Consequently, the otherwise perfectly ordered arrangement of carbon atoms in the diamond crystal structure is disturbed. Some structural defects absorb light in the visible spectrum—the portion of the electromagnetic spectrum that the human eye can perceive—producing different bodycolors in diamonds. The defects responsible for color in many diamonds have been well documented, with the majority associated with nitrogen (e.g., Collins, 2001; De Weerdt and Van Royen, 2001; Kanda, 2007; Shigley and Breeding, 2013).

Yellow is one of the most common bodycolors of diamond. The majority of yellow diamonds—known as “cape” diamonds in the gem trade—are colored by the N3 defect (with a structure of three nitrogen atoms surrounding a vacancy) and associated absorptions including the N2 defect (a vibronic transition of N₃V) (Breeding et al., 2020). Yellow color in a significant number of diamonds can also come from single substitutional nitrogen (C-center) or from the H3 defect (a vacancy between two nitrogen atoms). A subset of yellow diamonds also contain nitrogen yet are primarily colored by a broad absorption band centered at about 480 nm in the visible spectrum. The 480 nm absorption band has been tentatively attributed to substitutional oxygen (Gali et al., 2001; Hainschwang et al., 2020), and diamonds with this absorption feature are termed “480 nm band diamonds” in the gemological research community. This absorption band has only been observed in natural diamonds, and the defect(s) associated with it have not been produced in natural or synthetic diamonds by commercial treatments (Collins, 2001).

Among all yellow diamonds submitted to GIA in the last decade, <5% have been 480 nm band diamonds. These diamonds have not received much attention in the gem trade because of their relative scarcity. However, it is notable that the 480 nm absorption band is not restricted to diamonds with primarily yellow color but also occurs in a variety of colored diamonds, including some of the most valuable orange and chameleon diamonds. Chameleon diamonds are those with a reversible color-change property, typically turning from a green to a more intense yellow or orange color when they are heated mildly or kept in the dark for a significant period of time (Fryer et al., 1981, 1982; Hainschwang et al., 2005; Fritsch et al., 2007a; Fritsch and Delaunay, 2018). Exposure to ambient light or cool temperatures restores the original color. While the documentation of chameleon diamonds is sparse, it is possible that many of these diamonds have not been evaluated and recognized due to a lack of awareness of their properties. This article provides detailed descriptions of the gemological and spectroscopic properties of the diverse and dynamic 480 nm band diamonds to facilitate their identification in the gem and mining industries.

COLOR VARIETIES AND CORRESPONDING VISIBLE ABSORPTION SPECTRA

The broad absorption band with a maximum at approximately 480 nm extends from violet to green in the visible spectrum, producing a yellow to orange color in diamond. Depending on the range and intensity of this absorption band, 480 nm band diamonds can have a variety of tones and saturations (figure 1). Generally, diamonds with Fancy Vivid, Fancy Intense, or Fancy Deep orange, orangy yellow, or yellowish orange color grades have the strongest 480 nm absorption band (figure 2, A and B). In contrast, chameleon diamonds usually have a much weaker 480 nm absorption band (figure 2C) (Lai et al., 2024a). The cause of green color in chameleon diamonds differs from that in green or blue-green diamonds colored by the GR1 defects (neutral vacancies). GR1 defects can be produced by natural or artificial radiation, and produce a green bodycolor in diamonds that have a yellow hue (Breeding et al., 2018). The green color in chameleon diamonds has been attributed to a transmission window between 500 and 600 nm produced by a combination of the 480 nm absorption band and the additional broad absorption band extending from approximately 600 nm to the near-infrared region (Hainschwang et al., 2005; Fritsch et al., 2007a). The majority of chameleon diamonds also have an absorption peak at 415 nm corresponding to the N3 defect (figure 2C), whereas this absorption feature is far less common in non-chameleon 480 nm band diamonds.

The occurrence of additional structural defects can modify 480 nm band diamonds’ colors. For example, single substitutional nitrogen frequently occurs in these diamonds, which could add an orangy or brownish tint to their primary colors. In addition, vacancy clusters in the diamond structure are also a common cause of brown coloration in diamonds (Fisher et al., 2009). In some 480 nm band diamonds with dominant brown bodycolor there is a continuous absorption that increases from the red end of the visible spectrum and reaches a maximum near 500 nm (figure 2D). This increasing absorption may mask the 480 nm absorption band, increasing the difficulty of identifying 480 nm band diamonds based solely on their visible absorption spectra. In this case, other gemological and spectroscopic features are required to detect the existence of the 480 nm absorption band.

On very rare occasions, some non-chameleon 480 nm band diamonds contain absorption features typically observed in cape diamonds (figure 2E), with absorption peaks at 415 and 478 nm corresponding to the N3 and N2 absorptions, respectively (Altobelli and Johnson, 2014). Unlike chameleon diamonds (which frequently show N3 absorption in addition to the 480 nm absorption band), non-chameleon 480 nm band diamonds with N3 and N2 absorptions generally have yellow or orangy yellow bodycolors without a green component, and N2 absorption has not been observed in chameleon diamonds. Another group of uncommon 480 nm band diamonds contains an additional broad absorption band centered at about 550 nm and a minor absorption peak at 525 nm (Lall et al., 2024). This broad absorption band is the cause of the pink color associated with plastic deformation in most pink diamonds (Collins, 1982, 2001). Diamonds of this variety with a relatively stronger 480 nm absorption band generally have an orange color (figure 2F), whereas those with a more intense 550 nm band tend to have a pink color.

Irregular color zoning is often observed in 480 nm band diamonds when they are viewed microscopically—some zones have a more saturated color, while other zones have lower color saturation and/or are near-colorless. In some extreme cases, the color zoning can be distinct enough that a 480 nm band diamond will appear bicolored or even tricolored (figure 3). A notable example is a chameleon diamond with one half graded Fancy Dark orangy brown and the other half graded Fancy Dark brown greenish yellow; a color-change reaction was observed only in the brown–greenish yellow portion when the diamond was heated (figure 3, right; Lai and Eaton-Magaña, 2023).

MINERAL INCLUSIONS

Often minerals from mantle rocks are encapsulated by diamonds during growth. These mineral inclusions can be used to interpret the mantle rock from which they were derived, and hence the rock in which the diamond may have grown. Mineral inclusions observed in 480 nm band diamonds include pyrope-almandine-grossular garnet, omphacite, rutile, graphite, and sulfide minerals. Pyrope-almandine-grossular garnet ((Mg,Fe,Ca)₃Al₂Si₃O₁₂), omphacite ((Ca,Na)(Mg,Fe,Al)Si₂O₆), and rutile (TiO₂) are minerals associated with eclogite—a major host rock for diamond in the lithospheric mantle (figure 16). Based on the observation that most, if not all, inclusion-bearing 480 nm band diamonds submitted to GIA are eclogitic, it is inferred that these diamonds are more likely to be found in mines that predominantly produce eclogitic diamonds. However, this does not preclude the possibility that 480 nm band diamonds can also form in peridotite—another major diamond host rock in the lithospheric mantle comprised predominantly of olivine ((Fe,Mg)₂SiO₄), orthopyroxene ((Mg,Fe)₂Si₂O₆), and chromium-rich diopside ((Ca,Cr)MgSi₂O₆).

CONCLUSIONS

Diamonds colored by the 480 nm absorption band have attracted little attention in the mining industry and gem trade, due in part to their scarcity and therefore unfamiliarity with their properties. While most 480 nm band diamonds have a saturated yellow bodycolor similar to that generated by other diamond defects, these diamonds also occur in a variety of additional colors, including the highly sought-after pure orange diamonds and color-change chameleon diamonds.

Rapid diamond screening can be achieved by exposing them to long-wave UV, as 480 nm band diamonds generally emit medium to strong yellow fluorescence, a mixture of yellow and blue fluorescence, or (occasionally) orange fluorescence. The presence of micrometer-sized dark inclusion clusters—identified as graphite—are a diagnostic gemological feature of 480 nm band diamonds. These platy inclusions are extremely thin and highly reflective when viewed at certain angles, with rounded or well-defined hexagonal shapes. Other characteristic features of 480 nm band diamonds include irregular surface fluorescence patterns, anomalous absorption in the one-phonon region of the FTIR spectrum, and a broad PL band centered at 650–685 nm when the diamond is excited by lasers with wavelengths in the range of, for example, 488–532 nm.

Currently, no known commercial treatments or synthetic growth methods can create the defect(s) associated with the 480 nm absorption band or the color-change property associated with chameleon diamonds. However, it is certainly possible to artificially create other defects in 480 nm band diamonds to alter their colors (e.g., laboratory irradiation treatments to produce a green bodycolor); therefore, advanced gemological testing may be required to ensure that the diamonds are naturally colored. Finally, care must be taken when examining colorless or pink diamonds, as small yellow color zones caused by the 480 nm absorption band may occur in these diamonds and affect their color grades.