Prismatic Bicolor Dumortierite Crystal
Dumortierite is an orthorhombic, fibrous aluminum borosilicate mineral (Al7BO3(SiO4)3O3) that typically forms slender prismatic crystals with spectacular color and pleochroism. The crystals can vary in color from brown, blue, and green to rarer violet and pink. These color variations are caused by the substitution of iron and other trivalent elements for aluminum. Dumortierite typically occurs in high-temperature, aluminum-rich regional metamorphic rocks resulting from contact metamorphism as well as in boron-rich pegmatites, with several reported sources, including Austria, the United States (Nevada), Madagascar, and Russia. A few reports of the mineral’s occurrence have been published in the literature, which may be of general interest to mineralogists (e.g., A.B. Peck, “Dumortierite as a commercial mineral,” American Mineralogist, Vol. 11, No. 4, 1926, pp. 96–101). Because perfect dumortierite crystals are very rare, small crystals surrounded by quartz and andalusite are more typically observed. Dumortierite has been reported to commonly occur as a blue mineral inclusion in quartzite (Spring 2015 Gem News International, pp. 100–102).
Recently, GIA’s Bangkok laboratory received a 63.17 ct partially polished rough crystal with a prismatic shape and hexagonal cross section, measuring approximately 26.70 × 17.24 × 15.23 mm. The sample exhibited a uniform gray-blue color externally, with a strongly zoned bicolor core running along the length of the crystal featuring a pinkish orange center surrounded by a rim of gray-blue (see above). The color zoning was reminiscent of watermelon tourmaline’s appearance, but in a different crystal form and color. The bicolored zoning observed in this dumortierite may be the result of a change in trace element composition during the crystal’s growth.
A refractive index of 1.67–1.69, a hydrostatic specific gravity of 3.38, and Raman spectroscopy identified this stone as dumortierite. The stone displayed strong chalky green fluorescence throughout the surface under short-wave (254 nm) ultraviolet radiation. Microscopic observation revealed a strong, partially hexagonal growth feature along the crystal outline with a zoned pinkish orange core and a gray-blue outer layer. The partially hexagonal polished cross section displayed triangular sectors under cross-polarizing filters. Fluid inclusions and fingerprints as well as intersecting cleavage planes were also present in the stone.
As the cause of blue color in dumortierite has been previously reported to be mainly from Fe2+-Ti4+ intervalence charge transfer (A.N. Platonov et al., “Fe2+-Ti4+ charge-transfer in dumortierite,” European Journal of Mineralogy, Vol. 12, No. 3, 2000, pp. 521–528), energy dispersive X-ray fluorescence was also performed on the stone to obtain elemental information and to support its identification. High aluminum and silicon, which are among the main components of dumortierite, were detected along with small peaks of other elements, such as silver, titanium, chromium, and iron.
This dumortierite rough crystal is remarkable for its vivid, clearly separated bicolor zones and its rarity, and to the author’s knowledge, is the most significant example reported in the literature due to its size, color, and shape.
