Diamond and Related Materials 12 (2003) 330–335 0925-9635/03/$ - see front matter  2003 Elsevier Science B.V. All rights reserved. PII: S0925-9635 Ž 02 . 00307-2 Physical and chemical features of a large coated natural diamond crystal Wuyi Wang *, Thomas M. Moses , James E. Shigley a, a b GIA Gem Trade Laboratory, 580 Fifth Avenue, New York, NY 10036, USA a GIA Research, Gemological Institute of America, 5345 Armada Drive, Carlsbad, CA 92008, USA b Abstract A large (390.7 carat, 78.14 g) natural diamond crystal, composed of a near colorless gem-quality core and a translucent yellowish-green overgrowth coating approximately 0.5 mm in thickness, was investigated to better understand the relationship between the two portions of this crystal. The coating is translucent as the result of numerous tiny inclusions of trapped magmatic meltyfluid that originated in the Earth’s mantle. The nitrogen impurity is dominantly in the A form. Despite its ‘dirty’ outer appearance, the coating contains fewer defects and impurities in comparison to the core. The trapped melts are rich in SiO (63.7 2 wt.%), Na O (7.1 wt.%),KO (7.0 wt.%) and H O (5.9 wt.%). In terms of trace elements, significant amounts of Ba, Sr, Zr, 2 2 2 and the light rare earth elements are also present. No CO or carbonate component was detected; this is in contrast with analytical 2 results on other diamonds reported previously. Quartz is the only mineral that could be identified in the trapped melt. As indicated by the quartz peak shifts in the middle infrared region, these melt inclusions are confined at a pressure of ;1.7 GPa. All these results indicate that the coating is a secondary growth over a preexisting diamond. Growth of the coating very likely occurred at a diamond stable condition instead of meta-stable crystallization. The trapped melt is chemically different in terms of major and minor element compositions from kimberlitic magma, which transported most diamonds to the Earth’s surface.  2003 Elsevier Science B.V. All rights reserved. Keywords: Defects; Impurities; Optical properties 1. Introduction Natural diamonds occasionally contain other minerals as inclusions. Olivine, garnet and pyroxene (diopside) are minerals commonly occurring as inclusions in dia- mond. These inclusions (30–100 mm in size) usually show well-developed crystal shapes. Investigation of these inclusions is an important way to study the Earth’s mantle w1x. In addition to these ‘regular’ diamond crystals, much interest has also centered on the so-called ‘coated diamonds’. A coated diamond usually consists of a gem-quality core covered by a turbid overgrowth. The coating is usually translucent to opaque due to the presence of numerous micro-inclusions, with color var- iations from light- to dark-gray depending upon the nature and abundance of the inclusions. Previous studies have shown that the coatings have a fibrous internal structure w2,3x, and a thickness up to 1–2 mm. Minerals *Corresponding author. Tel.: q1-212-221-5858x3507; fax: q1- 212-719-3134. E-mail address: wuyi.wang@gia.edu (W. Wang). like quartz, mica, apatite, high-Ca and low-Ca carbon- ates, and biotite have been identified as inclusions in the coatings w4–6x. These micro-inclusions are sub- micrometer in size, usually smaller than 0.2 mm w4x. Coated diamonds have been found in kimberlites from Australia, Botswana, China, India, Russia and Zaire w6– 12x. The study of this type of diamond is important because it is the only available source of primary mantle fluidymelt. In addition, the mechanisms of coating growth and features of the coated diamond itself have attracted much scientific attention. In this study, we report on a typical but large coated diamond. Physical and chemical properties of both the coating and interior gem-quality core are documented. Based on these results, the formation of coated diamonds is discussed, as well as the possibility of using trace element chemistry to determine geographic source of gem diamonds. 2. Materials and methods The studied diamond is a twinned crystal (macle) weighting 390.70 carats (Fig. 1), from the Democratic