This comprehensive report explores the extraordinary attributes of dumortierite, with particular attention to an exceptionally uncommon and visually striking bicolor crystal. It investigates the mineral's genesis, its chemical makeup, and the distinctive qualities that render this specific sample an object of considerable scientific and aesthetic fascination within the domains of mineralogy and gemology.

Dumortierite, a borosilicate mineral of aluminum (Al₇BO₃(SiO₄)₃O₃), typically presents itself as slender prismatic crystals. Its palette encompasses shades of brown, blue, green, and occasionally violet or pink. These chromatic variations largely stem from the substitution of iron and other trivalent elements for aluminum within its crystalline lattice. This mineral commonly develops in environments characterized by high temperatures, abundant aluminum metamorphic conditions, or in pegmatites rich in boron. Notable deposits have been discovered in locales such as Austria, the United States (specifically Nevada), Madagascar, and Russia. While perfectly formed crystals are a rarity, it is more frequently observed as minute inclusions within quartz and andalusite, often appearing as a sapphire-hued inclusion embedded in quartzite.

A recent and significant gemological revelation concerns a 63.17-carat, partially polished rough crystal that arrived at GIA’s Bangkok laboratory. This specimen, boasting a prismatic structure and a hexagonal cross-section measuring approximately 26.70 × 17.24 × 15.23 mm, initially presented a uniform gray-blue exterior. However, its internal composition unveiled a remarkable and clearly defined bicolor core extending along its entire length: a pinkish-orange heart encased by a gray-blue periphery. This striking color stratification evoked comparisons to watermelon tourmaline, albeit within a distinct crystal architecture and chromatic arrangement. Such pronounced bicolored zoning in dumortierite suggests a considerable alteration in its trace element composition during the various stages of the crystal's development.

Through an exhaustive gemological examination, the crystal's identity as dumortierite was unequivocally confirmed. This was established by a refractive index ranging from 1.67 to 1.69, a hydrostatic specific gravity of 3.38, and corroborating Raman spectroscopy data.  Microscopic scrutiny revealed clear, partially hexagonal growth patterns along its outer perimeter, corresponding precisely to the zoned pinkish-orange core and the gray-blue exterior. A polished cross-section, which was also partially hexagonal, displayed triangular sectors when viewed under cross-polarizing filters. Furthermore, the stone contained various inclusions, including fluid pockets, fingerprint-like patterns, and intersecting cleavage planes, providing further insights into its formation history.

To gain a deeper understanding of its chemical makeup and to reinforce its identification, energy dispersive X-ray fluorescence (EDXRF) analysis was conducted. This advanced analysis confirmed high concentrations of aluminum and silicon, which are the primary elemental constituents of dumortierite. Additionally, trace quantities of other elements, such as silver, titanium, chromium, and iron, were also identified. Prior research has indicated that the characteristic blue coloration observed in dumortierite is largely attributable to an Fe²⁺-Ti⁴⁺ intervalence charge transfer mechanism. These findings provide crucial context for the elemental composition and elucidate their pivotal role in shaping the crystal's remarkable visual properties.

This rough dumortierite specimen is truly an extraordinary discovery, distinguished by its vibrant, sharply delineated bicolor zones and its exceptional scarcity. Its impressive dimensions, unique coloration, and distinctive morphology position it as arguably the most significant example documented in existing mineralogical literature. It offers an unparalleled specimen for both rigorous scientific inquiry and profound aesthetic appreciation, pushing the boundaries of what is known about this fascinating mineral.