Spring 2012 Lab Notes
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Editors Thomas M. Moses | Shane F. McClure DIAMOND laboratory recently examined two type Artificially Irradiated Type IIb IIb green brilliants, a 5.84 ct pear shape and a 0.30 ct round, that were color Saturated blue color is rare in natural graded as Fancy Dark gray-yellowish diamond, and various treatment methods green and Fancy Light yellow-green, have been developed to introduce or respectively (figure 2). enhance this effect. The more common Both were very clean microscopically. techniques include the annealing of type In cross-polarized light, the pear showed IIb diamonds under high-pressure/high- the tatami strain typical of a natural temperature (HPHT) conditions and the diamond. The round brilliant did not high-energy beam irradiation of light- exhibit any strain, but did show subtle colored type Ia/IIa diamonds. In the New color zoning (figure 3). DiamondView York laboratory, we recently examined a imaging of the pear shape revealed blue very rare case of a type IIb diamond artifi - luminescence with dislocations (straight cially irradiated to enhance its blue color. lines), indicating a natural diamond, This modified step-cut shield (13.70 Figure 1. The Fancy Deep green-blue while the round brilliant displayed a 10.75 5.09 mm) weighed 4.13 ct and ¥ ¥ color of this 4.13 ct type IIb diamond typical HPHT-synthetic growth pattern was color graded Fancy Deep green-blue is due to artificial irradiation. (figure 4). Spectroscopic analysis con - (figure 1). It displayed a clear color firmed a natural color origin for the pear concentration in the culet, an important and an as-grown color for the round visual indication of artificial irradiation. transmission window in the green–light brilliant. Both were verified as type IIb by Infrared absorption spectroscopy re - blue region. From these observations, the boron bands in their mid-infrared vealed a typical spectrum for a type IIb we confirmed that this diamond had spectra at ~2927 and ~2801 cm -1. diamond, with an intense 2800 cm -1 been artificially irradiated to improve Dislocations in a natural diamond peak corresponding to an optically its color. Strong plastic deformation occur during plastic deformation, which active boron concentration of ~40 ppb. indicated by high strain suggested that usually creates a brown color. In type A type IIb diamond with this size and the diamond had a significant brown IIb diamonds, the same process adds a boron concentration usually has a clear component before the treatment. This gray component to the blue color. In blue color (with a grayish or brownish also explains the strong green coloration this pear-shaped stone, however, plastic component, depending on the intensity observed after irradiation. deformation also contributed a yellow of plastic deformation) but not enough Type IIb diamonds are rarely irradiated component. The resulting combination saturation for a Fancy Deep grade. The to improve their color. This unusual of yellow and blue produced the 5.84 ct absorption spectrum in the UV-Vis sample allowed us the opportunity to diamond’s yellowish green bodycolor. region at liquid-nitrogen temperature examine the interaction of a vacancy Interestingly, the light yellow-green showed strong GR1 absorption and a defect (GR1) with other defects in a type synthetic diamond had a different cause weak 666.7 nm peak, resulting in a IIb diamond. of color. In addition to boron bands, a Wuyi Wang and Paul Johnson small amount of single substitutional nitrogen was detected at 1344 cm -1 in Editors’ note: All items were written by staff Type IIb Green, the mid-infrared spectrum. An earlier members of GIA laboratories. Natural and Synthetic study reported mixed type IIb + Ib syn - GEMS & G EMOLOGY , Vol. 48, No. 1, pp. 47 –52, Type IIb diamonds are typically blue, thetic diamonds with blue and yellow http://dx.doi.org/10.5741/GEMS.48.1. 47 . resulting from boron defects, and it is growth sectors (J. E. Shigley et al., “Lab- © 2012 Gemological Institute of America very unusual to see a distinct green color grown colored diamonds from Chat - in such diamonds. The New York ham Created Gems,” Summer 2004 LAB NOTES GEMS & G EMOLOGY SPRING 2012 47 G&G , pp. 128–145). The article pro - posed that the combination of these growth sectors produced a green or grayish green color in faceted samples. In the 0.30 ct synthetic diamond re - ported here, the same coloring mecha - nism—the com bined effect of a boron-dominated sector and an iso - lated-nitrogen sector—caused the yel - low-green bodycolor. In both samples, the cutting orientation was critical to the proper mixing of the blue and yel - low components. Therefore, other nat - ural and synthetic diamonds con taining Figure 2. These type IIb samples consist of a Fancy Dark gray-yellowish blue and yellow color com ponents may green natural diamond (5.84 ct, left) and a Fancy Light yellow-green syn - not show a green bodycolor. thetic diamond (0.30 ct, right). These type IIb specimens demon - strate that plastic deformation or a combination of boron and nitrogen defects can result in unexpected green coloration at the hand of a skilled diamond cutter. Kyaw Soe Moe With Unusual Color Zoning An optical center with a broad absorp - tion band at ~480 nm is occasionally observed in some natural yellow-orange diamonds, as well as in “chameleon” diamonds. Yet little is known about this feature’s atomic structure or its mecha - Figure 3. In cross-polarized light, the pear shape showed the tatami strain nism of formation in natural diamonds. found in natural diamond (left, magnified 30×), while the synthetic round In the New York laboratory, we recently brilliant did not feature any strain but did show subtle color zoning (right, encountered a particularly interesting magnified 55×). manifestation of this optical center. A 0.50 ct rectangular diamond (4.43 ¥4.29 ¥2.80 mm) was color graded Fancy Intense orange-yellow. Its absorption spectrum in the mid-infrared region showed moderate concentration of A- form nitrogen and some unassigned peaks. A strong absorption band at ~480 nm, detected in the UV-Vis spectrum at liquid-nitrogen temperature, appeared to be the cause of the intense orange- yellow color. An outstanding feature of this diamond, visible during microscopic examination, was its distinct color zoning. The orange-yellow color was concen - trated in parallel zones separated by near-colorless areas (figure 5). This banded color distribution was matched by the Figure 4. DiamondView imaging of the pear shape revealed blue lumines - diamond’s fluorescence reaction to cence and dislocations corresponding to a natural origin (left), while the long-wave UV radiation. The orange- round brilliant showed growth zoning indicative of an HPHT-grown syn - yellow color zones showed very strong thetic diamond (right). yellow -orange fluorescence, while the 48 LAB NOTES GEMS & G EMOLOGY SPRING 2012 Large EMERALD with Gota de Aceite Structure Gota de aceite (Spanish for “drop of oil”) is a transparent angular or hexag - onal growth structure rarely seen in emerald (R. Ringsrud, “Gota de aceite : Nomenclature for the finest Colom - bian emeralds,” Fall 2008 G&G , pp. 242–245). The New York laboratory had the opportunity to examine a 24.25 ct Figure 5. The orange-yellow color emerald showing this phenomenon in this 0.50 ct diamond is concen - (figure 7). trated in parallel zones separated Microscopic observation revealed by near-colorless bands. transparent growth structures with an oily appearance throughout the stone (figure 8, left). The effect could even be near-colorless zones displayed strong blue seen with the unaided eye. Some of the Figure 7. This 24.25 ct Colombian fluorescence (figure 6). Micro scopic obser - structures displayed six well-defined emerald showed the rare gota de vation with crossed polarizers showed arms intercalated with six growth aceite growth structure. little internal strain, and there was no sectors, forming a 12-sided outline observable strain variation between the (figure 8, right); others showed an different color zones. From these obser - angular outline without arms. These Colom bian emerald. Individual and vations and the well-known fact that the structures occurred as individuals or in compact groups of colorless, transpar - 480 nm center luminesces yellow-orange elongated groups. The c-axis of each ent prismatic inclusions were identi - to UV radiation, it became clear that the growth structure was parallel to the fied by Raman spectros copy as quartz 480 nm center was distributed with a optic axis of the host emerald. Such (see photo in the G&G Data Deposi - zoned structure. It was also obvious that columnar growth zoning may have been tory at gia.edu/gandg), a well-known this banded structure was not associated developed by the parallel growth of inclusion in Colom bian emerald but with plastic deformation, a very common numerous sub-crystals, which were not previously reported in gota de cause of color zoning in natural diamonds. overgrown by the host emerald (E. J. aceite specimens. The stone also con - While the origin of the unusual distri - Gübelin and J. I. Koivula, Photoatlas tained strong planar color zoning, as bution of the 480 nm center in this of Inclusions in Gemstones , Vol. 3, well as partially healed fissures and diamond is unknown, the skillful orien - Opinio Publishers, Basel, Switzerland, fractures that showed evidence of clar - tation of the color banding by the cutter 2008, pp. 43 3– 434). ity enhancement. produced a face-up appearance that The sample’s jagged two- and Viewed in diffused light, the emerald’s received a Fancy Intense color grade. three-phase inclusions and spectro - green color was clearly concentrated Marzena Nazz scopic features confirmed it was a within the growth structures described Figure 6.