Emmanuel Fritsch

An update on color in gems. Part I: Introduction and colors caused by dispersed metal ions

Studies concerning the origin of color in gem materials have grown in sophistication in recent years, so that much new information is now available about natural color and its possible modification by various treatment processes. This three-part series of articles reviews our current understanding of gemstone coloration. The first part summarizes the factors that govern the perception of color, from the source of light to the human eye, and then examines in detail the role of one color-causing agent, dispersed metal ions, in the coloration of many gem materials, including ruby and emerald. The second part will explore charge-transfer phenomena and color centers as the cause of color in gems such as blue sapphire and Maxixe beryl. The series will conclude with colors that can be explained using band theory and physical optics, such as the play-of-color in opal and the blue sheen of moonstone feldspars.

An Update on Color in Gems. Part 3: Colors Caused by Band Gaps and Physical Phenomena

The previous two articles in this series described the origins of color in gems that derive from isolated structures of atomic dimensions—an atom (chromium in emerald), a small molecule (the carbonate group in Maxixe beryl), or particular groupings of atoms (Fe^(2+)-O-Fe^(3+) units in cordierite). The final part of this series is concerned with colors explained by band theory, such as canary yellow diamonds, or by physical optics, such as play-of-color in opal. In the case of band theory, the color-causing entity is the very structure of the entire crystal; in the case of physical phenomena, it is of microscopic dimension, but considerably larger than the clusters of a few atoms previously discussed.

The Gemological Properties of the De Beers Gem-Quality Synthetic Diamonds

Gem-quali ty synthetic diamond crystals weighing up to 11 ct have been grown in limited numbers at the De Beers Diamond Research Laboratory since the 1970s. These crystals have been produced strictly on an experimental basis and are not commercially available. Examination of a group of 14 brown1s17 yellow, yellow, and greenish yellow synthetic diamonds reveals distinctive gemological properties: uneven color distribution, geometric graining patterns, metallic inclusions, and, i n most cases, fluorescence t o shortwave but not to long-wave U.V radiation.

The Gemological Properties of the Sumitomo Gem-Quality Synthetic Yellow Diamonds

The distinctive gemological properties of the gem-quality synthetic yellow diamonds grown by Sumitomo Electric Industries are described. These synthetic diamonds, produced on a commercial basis, are grown as deep yellow single crystals i n sizes up to 2 ct. The material i s currently marketed for industrial applications only, in pieces up to about 0.40 ct. The synthetic diamonds can be distinguished by their ultraviolet fluorescence (inert to long-wave; greenish yellow or yellow to short-wave); their unusual graining, veining, and color zonation under magnification; and the absence of distinct absorption bands in their spectra.

Common gem opal : An investigation of micro-to nano-structure

Abstract The microstructure of nearly 200 common gem opal-A and opal-CT samples from worldwide localities was investigated using scanning electron microscopy (SEM). These opals do not show play-of-color, but are valued in the gem market for their intrinsic body color. Common opal-AG and opal-CT are primarily built from nanograins that average ~25 nm in diameter. Only opal-AN has a texture similar to that of glass. In opal-AG, nanograins arrange into spheres that have successive concentric layers, or in some cases, radial structures. Common opal does not diffract light because its spheres exhibit a range of sizes, are imperfectly shaped, are too large or too small, or are not well ordered. Opal-AG spheres are typically cemented by non-ordered nanograins, which likely result from late stage fluid deposition. In opal-CT, nanograins have different degrees of ordering, ranging from none (aggregation of individual nanograins), to an intermediate stage in which they form tablets or platelets, to the formation of lepispheres. When the structure is built of lepispheres, they are generally cemented by non-ordered nanograins. The degree of nanograin ordering may depend on the growth or deposition rate imposed by the properties of the gel from which opal settles, presumably, fast for non-ordered nanograin structures in opal-CT to slow for the concentric arrangement of nanograins in the spheres of opal-AG.

Opals from Slovakia (“Hungarian” opals) a re-assessment of the conditions of formation

Slovakian opals are found in an andesitic host-rock and believed to have formed by water circulation during a tectonic event. Their physical properties are investigated: X-Ray Diffraction (opal-A), Raman spectra (main Raman peak at 437 cm −1 ) and microstructure (large silica spheres 125 to 270 nm in diameter) surprisingly are properties of opals usually found in sedimentary deposits, and differ from those of opals found in other volcanic deposits. The temperature is proposed to control these physical properties rather than the nature of the host-rock. Some preliminary results of oxygen isotopic composition indicate a high δ 18 O for Slovakian and Australian opals (≈ 31‰) consistent with low temperatures of formation (lower than 45°C); by contrast, Mexican opals-CT show a lower δ 18 O at 13‰ consistent with a formation at a higher temperature, possibly up to 190°C.

Identification of the Endangered Pink-to-Red Stylaster Corals by Raman Spectroscopy

RAPID COMMUNICATIONS GEMS & GEMOLOGY SPRING 2009 n June 2007, delegates from 171 countries convened at The Hague to decide which species to include under the CITES (Convention on International Trade in Endangered Species of Wild Fauna and Flora) agreement. The aim of CITES is to ensure that international trade in plant and animal specimens does not threaten their survival. The species covered by the convention are listed in three appendices, according to the degree of protection they need. Appendix I includes species threatened with extinction, where trade is permitted only in exceptional circumstances. Species in Appendix II are not necessarily threatened with extinction, but their trade must be controlled to avoid use that would threaten their survival. Appendix III contains species that are protected in at least one country that has asked other CITES parties for assistance in controlling the trade. The gemological significance of this triennial meeting is that corals from the Corallium genus, the most important of all gem coral species, were being considered for protection under Appendix II (CITES, 2008a). Ultimately, it was decided not to include them. More recently, on April 8, 2008, China, which now has domestic laws to protect these species, requested that CITES include four Corallium species (C. elatius, C. japonicum, C. konjoi, and C. secundum) under Appendix III (Fish and Wildlife Service, 2008). Meanwhile, the Stylasteridae family, which includes all Stylaster gem corals (e.g., figure 1), remained listed under Appendix II of CITES (as of January 18, 1990), which means a certificate issued by the management authority from the country (or state) of export is required (CITES, 2008b). Pink-to-red corals have been used for ornamental purposes for about 10,000 years (Liverino, 1989). According to Rolandi et al. (2005), there are two classes, Hydrozoa and Anthozoa, within the Cnidaria phylum (i.e., cnidarians) that have skeletons durable enough for use in gem materials and carvings. These two classes each contain a family (Stylasteridae and Coralliidae, respectively) that together yield the majority of pink-to-red coral species used for ornamentation (Pienaar, 1981; Rolandi et al., 2005; Smith et al., 2007). Most corals found in the IDENTIFICATION OF THE ENDANGERED PINK-TO-RED STYLASTER CORALS BY RAMAN SPECTROSCOPY

Separating Natural and Synthetic Rubies on the Basis of Trace-Element Chemistry

GEMS & GEMOLOGY Summer 1998 orrect gem identification is crucial to the gem and jewelry trade. However, accurate information on a gem’s origin rarely accompanies a stone from the mine, or follows a synthetic through the trade after it leaves its place of manufacture. Today, natural and synthetic rubies from a variety of sources are seen routinely (figure 1). Usually, careful visual observation and measurement of gemological properties are sufficient to make important distinctions (Schmetzer, 1986a; Hughes, 1997). In some cases, however, traditional gemological methods are not adequate; this is particularly true of rubies that are free of internal characteristics or that contain inclusions and growth features that are ambiguous as to their origin (Hänni, 1993; Smith and Bosshart, 1993; Smith, 1996). The consequences of a misidentification can be in the tens of thousands, and even hundreds of thousands, of dollars. Ruby is a gem variety of corundum (Al2O3) that is colored red by trivalent chromium (Cr3+). Besides Cr, most rubies contain other elements in trace amounts that were incorporated during their growth, whether in nature or in the laboratory. For the purpose of this article, we consider trace elements to be those elements other than aluminum, oxygen, and chromium. These trace elements (such as vanadium [V] and iron [Fe]) substitute for Al3+ in the corundum crystal structure, or they may be present as various mineral inclusions (such as zirconium [Zr] in zircon) or as constituents in fractures. The particular assemblage of trace elements (i.e., which ones are present and their concentrations) provides a distinctive chemical signature for many gem materials. Since the trade places little emphasis on establishing the manufacturer of synthetic products, this article will focus on how trace-element chemistry, as determined by EDXRF, can be used for the basic identification of natural versus synthetic rubies. It will also explore how EDXRF can SEPARATING NATURAL AND SYNTHETIC RUBIES ON THE BASIS OF TRACE-ELEMENT CHEMISTRY

Gem-Quality Cuprian-Elbaite Tourmalines from São José Da Batalha, Paraíba, Brazil

Unusually vivid tourmalines from the state of Paraiba, in northeastern Brazil, have attracted great interest since they first appeared on the international gem market in 1989. This article describes what is known of the locality at this time, but focuses on the most striking characteristic of these gem tourmalines: the unusual colors in which they occur. Quantitative chemical analyses revealed that these elbaite tourmalines contain surprisingly high concentrations of copper, up to 1.92 wt.% Cu (or 2.38 wt.% CuO). Their colors are due to Cu^(2+) or a combination of Cu^(2+), Mn^(3+), and other causes. Some colors can be produced by heat treatment, but most also occur naturally.

Relationship between nanostructure and optical absorption in fibrous pink opals from Mexico and Peru

Translucent pink opals from Mexico (states of Mapimi and Michoacan) and Peru (Acari area, near Arequipa) are opal- CT, containing from 10 to 40 % palygorskite, as demonstrated by XRD, infrared absorption and specific gravity measurements. Their nanostructure is unusual, with bunches of fibres 20 to 30 nm in minimum diameter, related to the fibrous nature of paly- gorskite crystals, as demonstrated by electron microscopy. A complex absorption centred at about 500 nm is the cause of the pink colour. It is proposed that the absorption is due to quinone fossil products associated with the phyllosilicate fibres. The Raman spectrum of monoclinic palygorskite is deduced from that of its mixture with opal. The opal-CT-palygorskite-quinone associa- tion is a geological marker of a specific environment, presumably of a fossil lake environment in a volcanic region.