World Of Gems Conference 2017 Proceedings Book

WORLD OF GEMS CONFERENCE V - 6 - SEPTEMBER 2017 gence” is perceived. The best gem-quality synthetics tend to have far less crystallographic defects than natural stones. Thus they may show specific patterns (such as the “brush” pattern for the CVD) or may be truly isotropic, which has been observed for a number of years for some HPHT-grown specimens. Figure 2. Finally, the last resorts are two fairly expensive lumines- cence-based methods: The first is luminescence imaging. The observation of lumines- cence zoning, under ade- quate magnification is usually conclusive. It necessitates ex- pensive specialized instru- ments (for example the DiamondView) and also, a good knowledge of the very varied patterns of natural and synthetic diamond growth, to avoid mistaking a natural for a synthetic. The typical “step flow growth” of CVD synthet- ics is becoming less marked. The second is luminescence spectroscopy. This typically involves laser excitation with several lasers (mostly in the green, but also in the UV and red), at low temperature (liq- uid nitrogen). Further, this implies a ver y thorough knowledge of natural and synthetic diamond emis- sions, which limits this approach to a small number of labo- ratories. For example, Ni-related bands are indicative of a HPHT-grown diamonds (because of the Ni-containing flux) and Si-V bands more indicative of CVD growth. In practice, for a small number of stones, the definitive interpretation of spectra may remain a matter of discussion. This PL spectroscopy approach is the only method consid- ered reliable for the identification of HPHT-treated near-col- orless diamonds. The criteria are more sophisticated, implying relative proportions of emission bands (NV° and NV - ) and also their width. Yet, sometimes, suspected dia- monds show very little or no emissions: this means they have very few defects, which can be true of natural, syn- thetic or treated near-colorless diamonds. The separation of natural from synthetic diamond, especially melee, may appear very challenging. Yet, there are a limited number of simple devices which can help the jeweler-gemol- ogist, even more the diamond professional. The last resort appears to be luminescence spectroscopy, but it is expensive and a great deal of interpretation is required. Automated in- struments offer only limited help unless one is dealing with very large quantities. u Links of interest for synthetic diamonds: Chinese HPHT for abrasive grit. Typical equipment and procedure https://www.youtube.com/watch?v=PuB6OKdgAjY “Heart in a diamond” video. Growth of a diamond with a split sphere apparatus from 2:30 to 6:12 https://www.youtube.com/ watch?v=CqPEZTWfnHU HPHT sintering toroid (but for polycrystalline materials, not gem diamond) https://www.youtube.com/ watch?v=kO6shcIL2sg Algordanza. Brief apparition of the toroid machine 1:30 to 1:50 https://www.youtube.com/ watch?v=Atqj8PkpWf8 CVD Russ Hemley Carnegie INtitution Washington DC https://www.youtube.com/ watch?v=A4_l3pKhaJo SCIO diamond https://www.youtube.com/ watch?v=iQXVya-HAOk NOVA on synthetic diamonds, in two parts: in particular interesting movie of the CVD process at the atomic level around 7.50 in the first part https://www.youtube.com/watch?v=021v4BsNyZ4 https://www.youtube.com/watch?v=FglCni2_g1g&t=131s Nearly 155 ct single crystal CVD diamond wafer https://www.youtube.com/watch?v=WBCHnmXXink Mscreen sorting machine HRD http://www.hrdantwerp.com/en/product/m-screen ASDI sorting machine GGTL http://www.ssef.ch/products/ssef-instruments/asdi/ SYNTHETIC/TREATED DIAMONDS FIGuRE 2. Anomalous double refringence of synthetic diamonds: a HPHT- grown of high quality is nearly isotropic, hence appears all black as seen in top left inset. A CVD-grown synthetic shows the typical brush pattern seen here.