Fall 2000 Gems & Gemology - Gemfrance

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Figure 10. Because of the greater sensitivity and improved resolution provided by cryogenic cooling, these representative lowtemperaturephotoluminescence spectra show many features that were not recorded under room-temperature conditions. Again, the diamonds were analyzed before and after HPHT processing, using three different laser sources: (A) 244 nm, (B) 325 nm, and (C) 514/532 nm. See text for descriptions of the specific features. all. Even under these conditions, the GGL sample still did not show either of the N-V centers (574.8 or 637.0 nm). We recorded a distinct reversal in the ratio between the 574.8 and 637.0 nm peaks, as described by Fisher and Spits (2000). In our samples under these conditions, they ranged from 0.3 to 0.7. The FWHM of the 637.0 nm peak also increased slightly to approximately 0.64 to 1.00 nm. The slight broadening of this ZPL also occurred in all samples. RESULTS: CATHODOLUMINESCENCE AND X-RAY TOPOGRAPHY (See Box B) Cathodoluminescence Colors Before: We observed CL colors ranging from a chalky yellow-white of moderate intensity, to a strong blue-white. Most of the samples displayed an even texture, with no structure to the luminescence visible. However, one did reveal a slightly irregular or “cottony” overall texture, as well as a narrow “vein” that was slightly less luminescent. After: All samples revealed a general shift in the CL colors toward blue. The strongest shift occurred with samples that changed from a moderate yellow-white to a strong blue-white after processing (figure 11). We noted no change in the distribution of the CL texture. Cathodoluminescence Phosphorescence All the samples revealed a rapid and steadily declining phosphorescence, which lasted approximately 1–1.5 seconds, both before and after HPHT processing. GE POL Diamonds GEMS & GEMOLOGY Fall 2000 205
BOX B: CATHODOLUMINESCENCE ANDX-RAY TOPOGRAPHY AS NONDESTRUCTIVE TOOLS IN GEMOLOGY Cathodoluminescence (CL) spectroscopy and X-ray topography can provide a great deal of information about the structure of a diamond. Because HPHT procedures may heal the dislocations and lattice defects that produce certain luminescence centers, these two methods are useful in reconstructing the processes by which the lattice is changed. Although neither method is broadly applied in gemology, CL has been used extensively in technical studies of diamonds (Panczer et al., 1996). CATHODOLUMINESCENCE CL is the emission of light from a solid surface when excited by an electron beam. Depending on the accelerating voltage (usually between 1 and 30 kV), the electrons penetrate about 1–3 µm. Some fundamental properties of minerals—such as lattice defects, impurities, and other disturbances in the crystal lattice—are represented by luminescence centers. The energy of the electron beam is transferred within a diamond by these optical centers and can give rise to distinctive CL colors and other patterns. Today, two types of CL equipment are used for gemstones to detect these optical signals. “Cold CL” : Using flood gun optical microscopy, the luminoscope is mounted on a microscope stage. The electrons are generated in a cold cathode device. This small glass tube contains discharge gas (e.g., air, nitrogen, or helium) as well as the cathode and the anode. As soon as high tension is applied between the cathode and the anode, electrons are created. The electrons pass through the hollow anode and enter the low-evacuated (10 –2 Torr) sample chamber. A lead-glass window in the sample compartment allows visual observation of the luminescence behavior, so the CL color, zonation, and phosphorescence can be observed. Color photomicrographs of these features can be taken at magnifications up to 125×. The light emitted by the diamond is focused via the microscope objective onto a monochromator. Luminescence spectra can be recorded within a range of 380 to 1000 nm (compare to Ponahlo, 1996). “Hot CL”: Hot-cathode luminescence microscopy is relatively new (Götze, 1996). The CL spectrometer is attached to a scanning electron microscope (SEM), and the electrons are generated by a hot filament. The typical acceleration voltage is 15 kV, and the beam current is about 1 nA. The sample must be mounted on a special holder because of the high vacuum (about 10 –6 Torr) within the sample compartment. In addition, the diamond must be coated by graphite, both so the SEM can focus on the surface of the sample (at magnifications up to about 100,000×) and to avoid charge clouds generated by the electron beam. This equipment can combine the optical and chemical analytical capabilities of the SEM-EDS system. Because the electron beam of the SEM is focused, very small areas (
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Page 4 and 5: Pearl Nucleation Misquote? It is di
Page 6 and 7: Figure 1. These two illustrations s
Page 8 and 9: When a type IIa diamond is exposed
Page 10 and 11: GE5 GGL1 2.72 ct 0.61 ct Before Aft
Page 12 and 13: after processing revealed little ch
Page 14 and 15: Figure 7. These UV/Vis/NIR absorpti
Page 16 and 17: A couple of samples also revealed p
Page 20 and 21: Figure 11. The CL color of the GE P
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Page 26 and 27: BOX C: RUSSIAN HPHT-PROCESSED DIAMO
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Page 66 and 67: REFERENCES Arem J.E. (1987) Color E
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Figure 2. The laboratory recently e
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Figure 5. These round brilliants (t
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Figure 9. This 2.34 ct heated rough
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De Beers’s newly stated direction
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Aquamarine from southeast India. Fi
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Natural History, New York) and S. S
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& Gemology, pp. 78-89). The massive
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fluorescent light; for this sample,
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groups found in organic matter such
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Gemological ABSTRACTS EDITOR A. A.
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Tertiary-age diamondiferous fluvial
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JEWELRY RETAILING Mistakes jewelers
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Irradiation at doses of 7.6-187 kGy
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Fall 2000 Gems & Gemology - Gemfrance

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