The First Transparent Faceted Grandidierite, From Sri Lanka

Abstract

randidierite, a magnesium-aluminum borosilicate, MgAl 3 BSiO 9 , is mentioned only rarely as a "possible" blue gemstone (Ostwald, 1964), although the existence of faceted stones has been reported occasionally in the gemological literature (see, e.g., Mitchell, 1977). However, the samples available to date as faceted stones or cabochons, all originating from southern Madagascar, are opaque-never transparent-or at best translucent (Arem, 1987). This article describes the first transparent faceted grandidierite, which is also the first faceted grandidierite reported from Sri Lanka.Grandidierite occurs in pegmatites, contact metamorphic rocks (hornfelses), and high-grade (granulite facies) metamorphic rocks; worldwide about 40 localities are known (Grew, 1996). Grandidierite was first discovered in a pegmatitic environment at Andrahomana near Taolanaro (formerly Fort Dauphin) in southeastern Madagascar (Lacroix, 1902(Lacroix, , 1904. The orthorhombic mineral was named by Lacroix after Alfred Grandidier, one of the French explorers of Madagascar. Lacroix described the nontransparent material as both massive (1902) and forming crystals as long as 8 cm (1904). The color of grandidierite from different localities is described as blue, greenish blue, blue-green, and bluish green.The Kolonne area of Sri Lanka, which is located approximately 8 km south-southeast of Rakwana, near Ratnapura, has become known for some rare gem materials such as sapphirine (Harding and Zoysa, 1990), olivine with a high iron content (Burford and Gunasekera, 2000), and-most recently-the Ca-Mg-Al borosilicate serendibite (Schmetzer et al., 2002). In 2000 while in Ratnapura, one of the authors (MB) purchased the grandidierite described in this article, reportedly from the Kolonne area, as an 0.85 ct crystal fragment. After faceting, the rough yielded a 0.29 ct greenish blue transparent gemstone (figure 1).The Sri Lankan seller offered the rough gem (on the basis of its color) as a "possible serendibite." The original flat, tabular crystal did not appear to be water worn. Probably it was mined from a primary deposit, as is the case for most sapphirine from high-grade metamorphic host rocks in the Kolonne area (see again Harding and Zoysa, 1990). Based on some initial testing, the owner believed that the stone might be the rare gem mineral grandidierite and sent it to the senior author (KS) for additional tests and confirmation. MATERIALS AND METHODSThe faceted grandidierite was tested by standard gemological methods for refractive indices, optic character, specific gravity, and fluorescence to longand short-wave ultraviolet radiation, as well as the

Poudretteite: A Rare Gem Species from the Mogok Valley

“…Gemologists continue to be excited by the discovery of minerals that have never before been seen in a transparency and size suitable for faceting (see, e.g., McClure, 2002;Schmetzer et al, 2003). When such rarities are encountered, they present unique opportunities to expand the science of gemology and mineralogy.…”