Energy from natural radiation (α-, β-, and γ-particles) in the earth's crust displaces carbon atoms in the diamond lattice.This creates vacancies, including the General Radiation 1 (GR1) defect, that absorb light in the red portions of the visible spectrum and manifest as green body color and surface "stains."The intra-crystal distribution of interstitials, displaced from their normal lattice sites, is not well understood and may contribute to green color.In this study we apply Fourier transform infrared (FTIR) and photoluminescence (PL) spectroscopy to 10 natural Type Ia (FTIR detectable nitrogen) green-stained diamonds.For all diamonds we determine nitrogen concentrations and acquire PL emission spectra for exteriors and interiors (≥ 20 μm deep) exposed by mechanical fracturing.We document the depth distribution of interstitial defects and use multivariate statistics to evaluate the data.The resulting dataset enhances our understanding of the processes influencing green color in natural diamonds.Geologically, radiation stains are significant because they record direct interaction between a diamond and its environment, including changing geological conditions during residence in the crust.It is generally accepted that the defects related to green radiation stains form on the diamond surface ≤ 20 μm deep due to direct exposure to high-energy α-particles.In contrast, using a 457 nm laser we find that 484.4 nm peak intensity > 20 μm deep in some green-stained diamonds is stronger than on the surfaces of stain-free diamonds.Raman-normalized 484.4 nm peak intensity decreases across the surface and with depth away from green radiation stains.This indicates radiation damage beyond the penetration depth of α-particles.Other peaks including TR12 (470 nm) are intense on and directly adjacent to radiation stains, but are much weaker or undetected > 20 μm from a radiation stain.The TR12 defect in Type Ia diamonds relates to local defect structures possibly associated with α-particle exposure.
Among fancy-color diamonds, natural-color green stones with saturated hues are some of the rarest and most sought after.These diamonds are colored either by simple structural defects produced by radiation exposure or by more complex defects involving nitrogen, hydrogen, or nickel impurities.Most of the world's current production of fine natural green diamonds comes from South America or Africa.Laboratory irradiation treatments have been used commercially since the late 1940s to create green color in diamond and closely mimic the effects of natural radiation exposure, causing tremendous difficulty in gemological identification.Compounding that problem is a distinct paucity of published information on these diamonds due to their rarity.Four different coloring mechanisms-absorption by GR1 defects due to radiation damage, green luminescence from H3 defects, and absorptions caused by hydrogen-and nickel-related defects-can be identified in green diamonds.Careful microscopic observation, gemological testing, and spectroscopy performed at GIA over the last decade allows an unprecedented characterization of these beautiful natural stones.By leveraging GIA's vast database of diamond information, we have compiled data representative of tens of thousands of samples to offer a look at natural green diamonds that has never before been possible. In Brief• Among natural-color diamonds, those that have a pure green hue are rare and often highly valued.• While many green diamonds owe their color to natural radiation exposure, three other color causes are often observed.• These four categories of green diamonds exhibit some distinctive gemological properties and spectral features.• Separation of some natural-and treated-color green diamonds continues to present a challenge for gemtesting laboratories.
S ynthetic nano-polycrystalline diamond (NPD) may be one of the most important developments in synthetic diamond production in recent years.This transparent brownish yellow material is produced not by CVD or traditional HPHT synthesis methods, but rather in a multi-anvil press by a sintering process that converts high-purity graphite directly into synthetic diamond.According to the developer of the process, the conversion time averages 10-20 minutes (though can be less than 10 minutes) at 15 gigapascals and 2,300-2,500°C (T.Irifune, pers.comm., 2012).These pressures and temperatures are far higher than those used in the HPHT syn-thesis of single-crystal diamond.The material consists of randomly oriented nanoscale-sized synthetic diamond crystallites that have been bonded tightly together to form what may be thought of as an ultrahard synthetic diamond ceramic.A recent article by Skalwold (2012) documented a transparent 5 mm NPD sphere, and served as means to introduce this new synthetic diamond's development and some of its properties.Soon after that article was published, Dr. Tetsuo Irifune, director of Ehime University's Geodynamics Research Center (GRC), once again offered one of the authors (EAS) an exclusive opportunity to study this unique material.This time the specimen was a 0.88 ct faceted round brilliant (figure 1).In collaboration with researchers at GIA, an analysis of its gemological and spectroscopic properties was undertaken to establish gem identification criteria.
This article suggests new terminology to describe the banding of agates, in order to alleviate the confusion caused by previous descriptors that have no direct correlation to genetic implications.One ambiguous term, Uruguay banding, has been used to describe the straight parallel banding that often occurs in the lower portions of agate-containing vesicles from continental flood basalts.Confusion results because agates with this type of banding are called Uruguay agates, a term that some use to describe any agate from Uruguay.The author suggests using gravitational banding to refer to all agate textures caused by the force of gravity, which in this case applies to the deposition of relatively thick bands of coagulated silicic acid.The term adhesional banding is suggested as a replacement for terms such as concentric, common, normal, and fortification banding; these all refer to the thin layers of silica that adhere to the vesicle walls and form concentric rings or zones.Both types of banding commonly occur in agates formed in continental flood basalts from many locations worldwide.Several factors, including the amount and thickness of the lava flow, the temperature and humidity of the region, and the amount of CO 2 in the atmosphere, all contribute to the formation of the agate-filled vesicles.A suitable lava thickness will facilitate a slow cooling rate, which allows for the coalescence of numerous gas bubbles into larger vesicles and voids.Conversely, cooler atmospheric temperatures will inhibit vesicle formation.Sufficient rainfall will provide enough water for Gemological
Abstract Recent work in Barrovian metamorphic terranes has found that rocks experience peak metamorphic temperatures across several grades at similar times. This result is inconsistent with most geodynamic models of crustal over‐thickening and conductive heating, wherein rocks which reach different metamorphic grades generally reach peak temperatures at different times. Instead, the presence of additional sources of heat and/or focusing mechanisms for heat transport, such as magmatic intrusions and/or advection by metamorphic fluids, may have contributed to the contemporaneous development of several different metamorphic zones. Here, we test the hypothesis of temporally focussed heating for the Wepawaug Schist, a Barrovian terrane in Connecticut, USA, using Sm–Nd ages of prograde garnet growth and U–Pb zircon crystallization ages of associated igneous rocks. Peak temperature in the biotite–garnet zone was dated (via Sm–Nd on garnet) at 378.9 ± 1.6 Ma (2σ), whereas peak temperature in the highest grade staurolite–kyanite zone was dated (via Sm–Nd on garnet rims) at 379.9 ± 6.8 Ma (2σ). These garnet ages suggest that peak metamorphism was pene‐contemporaneous (within error) across these metamorphic grades. Ion microprobe U–Pb ages for zircon from igneous rocks hosted by the metapelites also indicate a period of syn‐metamorphic peak igneous activity at 380.6 ± 4.7 Ma (2σ), indistinguishable from the peak ages recorded by garnet. A 388.6 ± 2.1 Ma (2σ) garnet core age from the staurolite–kyanite zone indicates an earlier episode of growth (coincident with ages from texturally early zircon and a previously published monazite age) along the prograde regional metamorphic T – t path. The timing of peak metamorphism and igneous activity, as well as the occurrence of extensive syn‐metamorphic quartz vein systems and pegmatites, best supports the hypothesis that advective heating driven by magmas and fluids focussed major mineral growth into two distinct episodes: the first at c. 389 Ma, and the second, corresponding to the regionally synchronous peak metamorphism, at c. 380 Ma.
A new model is proposed for the formation of opal showing play-of-color, as well as potch (common opal).According to this new model, the essential requirements for opal formation are: (1) artesian "mound" springs with alkaline, silica-rich waters; (2) a mechanism for changing the physicochemical features of this water so that suitable silica spheres are precipitated in linear chains; and (3) the occurrence of suitable voids lined with clay-which acts as a semipermeable membrane to concentrate and purify the silica solution by ultrafiltration and dialysis.Active and extinct artesian mound springs are found in general proximity to several sedimentary rock-hosted opal fields in New South Wales, South Australia, and Queensland.In the Great Artesian Basin, the natural springs lie on a NEtrending line over 300 km long, subparallel to the Lightning Ridge opal fields.The lepispheres in play-of-color opal are very uniform in size (apparently within 2-3% of the mean size), indicating a batch process rather than a continuous one.The mixing of the high-pH spring water with cool, slightly acidic groundwater that has low total dissolved salts would decrease the pH, lower the temperature, and lower the ionic strength of the spring water; all three processes facilitate the formation of silica spheres.Montmorillonite (clay) can act as a semipermeable membrane and assist in the pressurization of the fluids for ultrafiltration.
critical to evaluating the relationships between diamond growth, color (e.g., figure 1), and response to laboratory treatments.With the increasing availability of treated and synthetic diamonds in the marketplace, gemologists will benefit from a more complete understanding of diamond type and of the value this information holds for diamond identification.Considerable scientific work has been done on this topic, although citing every reference is beyond the scope of this article (see, e.g., Robertson et al., 1934Robertson et al., , 1936;; and Kaiser and Bond, 1959).Brief gemological discussions of diamond types appeared in Shigley et al. (1986), Fritsch and Scarratt (1992), and Smith et al. (2000), and more-detailed descriptions were given in Wilks andWilks (1991) and Collins (2001).Nevertheless, repeated inquiries received at GIA indicate that many practicing gemologists do not have a clear understanding of the basics of diamond type.This article offers a readily accessible, gemology-specific guide to diamond type and related THE "TYPE" CLASSIFICATION
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