V ietnam, with an area of 335,000 km 2 , occupies the eastern side of the Indochinese peninsula. Most of the country's northern and central regions are mountainous, reaching an elevation of 3,142 m in the Fan Si Pan massif, near the Chinese border. The country is endowed with some 70 gem deposits and 160 different occurrences (Nguyen et al., 1995). Present gem production includes ruby, sapphire, spinel, tourmaline, peridot, garnet, aquamarine, topaz, quartz, and green orthoclase (e.g., figure 1). With more than 3,400 km of coastline, the country is also a source of saltwater cultured pearls, and several farms have emerged in recent decades. In addition, Melo pearls are retrieved by fishermen on the southern coast and in Ha Long Bay in the north. Compared with the country's gem wealth, however, the Vietnamese mining industry remains undeveloped. Although it has been nearly 30 years since colored stones were discovered in Vietnam, mining and pearl farming activities are mostly small-and medium-scale operations run by private individuals or small companies. In March 1988, state-owned Vinagemco (Vietnamese Gems Company) was established to direct the exploration, mining, and trading of gem materials (Kane et al., 1991; Pham et al., 2004b). Two subsidiaries, Yen Bai Gemstone Company and Nghe An Gemstone Company, were set up in those provinces that same year. Ultimately, management problems led to the company's downfall in July 2003. Since then, no state-owned company has been active in the gem sector. Mining, processing, cutting, and trading are all organized by private and joint-stock companies or private individuals. Scientific investigations of Vietnamese gem materials, including their properties and the genesis of the deposits, have resulted in several publications, with special attention to ruby and sapphire (e.g.,

Primary and secondary deposits in the Tan Huong-Truc Lau area of northern Vietnam's Yen Bai Province have supplied rubies (especially star rubies) and some sapphires-mostly of cabochon quality-for more than a decade. The gems are typically translucent to semitransparent and pink to purplish or brownish red, with rare color zoning. The most distinctive features of this corundum after polishing include growth zoning and asterism. The samples contain relatively high amounts of Fe and variable Ti and Cr. The geologic origin and gemological properties of this corundum are distinct from that of the adjacent Khoan Thong-An Phu area. For more than 20 years, Vietnam's Yen Bai Province, and the Luc Yen District in particular, have been widely known for producing highquality rubies and sapphires. Subsequent deposits found elsewhere in Vietnam (Long et al., 2004) include Quy Chau-Quy Hop (Nghe An Province), Di Linh (Lam Dong), Dak Ton (Dak Nong), and Ma Lam and Da Ban (Binh Thuan). Still, Yen Bai remains the country's most important source of ruby and sapphire. Much of the production consists of cabochonquality stones from the Tan Huong-Truc Lau area (e.g., figure 1), as described in this article.Vietnamese geologists first discovered gem-quality corundum and spinel at Luc Yen in 1983 (Vinh, 1991). In early 1987, the Geological Survey of Vietnam found abundant gem material in alluvium in Luc Yen's Khoan Thong area. Mining activity soon thrived, with many companies operating in the region (Voi, 1991). Other gem occurrences near Khoan Thong followed, such as Nuoc Ngap, Hin

dealers, ~100-150 kg of gem material were recovered by local people annually during the past decade, and in 2010 some 300-400 kg were produced. A limited number of gemstones have been faceted so far (e.g., figure 1), and sold mainly into the Vietnamese market. This article describes the geologic setting, gemological properties, and spectroscopic characteristics of Thuong Xuan aquamarine. LOCATION, GEOLOGY, AND MININGThe Thuong Xuan aquamarine deposits lie ~70 km west of the provincial capital, Thanh Hoa City (figure 2). While Thuong Xuan is the only commercially significant aquamarine locality in the country, the adjacent Que Phong District in Nghe An Province has produced small amounts of aquamarine from eluvial deposits.The Thuong Xuan region is cross-cut by a group of mainly northwest-trending faults. The aquamarine is hosted by pegmatites distributed mostly within the Ban Chieng and Ban Muong granite complexes, which together cover an area of 100 km 2 (figure 3). The pegmatite bodies typically form lenses or veins, ranging from 10 to 30 cm thick and a few meters in length to 4-5 m thick and tens of meters long. According to unpublished research by one of the authors (NNK), the pegmatites consist of quartz (38-48%), K-feldspar (~35%), plagioclase (18-24%), muscovite (2.3-3.5%), and biotite (0-2%). Quartz, feldspar, aquamarine, topaz, tourmaline, fluorite, and (rarely) zircon are found in miarolitic cavities. Although the topaz is of gem quality, it is not hosted by the same pegmatites as the aquamarine.Many eluvial occurrences of aquamarine are known, of which four-Ban Pang,

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