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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

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