Iridescent Tubes in Pezzottaite

Rhiana Elizabeth Henry, Taryn Linzmeyer, and Jeffrey Hernandez

A large 8.13 ct emerald-cut purplish pink pezzottaite (CsLiBe₂Al₂Si₆O₁₈) was submitted to the Carlsbad laboratory for gem identification (figure 1). Pezzottaite is a member of the beryl mineral group. This specimen had a measured refractive index of 1.600–1.610 and a specific gravity of 3.06. This stone was exceptional for its large size and quality, and gemological testing revealed no evidence of clarity treatment. The identity of the specimen was verified using Raman and Fourier-transform infrared spectroscopy, which also revealed the presence of H₂O in the crystal structure channels of the specimen, a common feature in pezzottaite. While the clarity of this pezzottaite was impressive for this mineral, especially without enhancement, the overall transparency was diminished by the presence of inclusions and small fractures. Some of these inclusions, such as the growth bands and tubules, could be considered distinctly beautiful features. Although the stone itself had a purplish pink hue, in darkfield illumination it appeared dark pink-purple (figure 2). The sample exhibited faint growth banding, with thin tube inclusions approximately perpendicular to the growth banding (i.e., tubes imperfectly parallel to the c-axis, crossing at shallow angles, and banding nearly perpendicular to the c-axis). At some viewing angles, the tube inclusions displayed vivid iridescence ranging from magenta, purple, neon blue, and green, to neon yellow and orange, given appropriate illumination. The iridescence was more vivid in darkfield illumination, but brightfield lighting showed a pinkish orange background which made the tubes shimmer (figure 3). Although colorful in specialized microscope lighting, the tubular inclusions were semitransparent white to the unaided eye in common lighting conditions.