Rare Gadolinite Gemstone
GIA’s Tokyo laboratory recently received a 22.33 ct black faceted stone measuring 18.19 × 13.05 × 11.05 mm (figure 1). The stone was nearly opaque, and it was difficult to observe internal inclusions and growth features. Numerous fractures and reflective fingerprints were observed over the entire surface, which showed a very dark green bodycolor with LED fiber-optic lighting under magnification. Metallic minerals with a yellow color were exposed on the surface along fractures (figure 2). The stone had a hydrostatic specific gravity of 4.25. The refractive index ranged from 1.770 to an over-the-limit reading with 1.81 RI liquid, giving a birefringence of at least 0.040.
Raman spectroscopy using 532 nm laser excitation showed a broad band around 900 cm–1 and a match to the RRUFF reference spectrum for gadolinite-(Y) (B. Lafuente et al., 2015, https://rruff.info/about/downloads/HMC1-30.pdf). Laser ablation–inductively coupled plasma–mass spectrometry was performed, revealing that the stone contained ytterbium, iron, beryllium, silicon, and rare earth elements (REEs) such as dysprosium and erbium, which is characteristic for gadolinite. The stoichiometric composition of the stone was calculated to be Y = 1.52, the sum of other REEs = 0.42, Fe = 0.80, Be = 1.82, and Si = 2.23, assuming a stoichiometric amount of O = 10. This result closely matched the formula of gadolinite and identified gadolinite-(Y) (Y2Fe2+Be2(Si2O10)) as the predominant composition. The stone’s standard gemological properties, Raman spectroscopic features, and stoichiometry indicated gadolinite-(Y).
Gadolinite-(Y) is a rare mineral species that occurs in metasomatically altered acidic igneous rocks or complex alkaline granite pegmatites (L. Nasdala et al., “Metamict gadolinite-(Y) from Ratnapura, Sri Lanka,” Journal of the Geological Society of Sri Lanka, Vol. 24, No. 1, 2023, pp. 23–30). Gadolinite can also be obtained from sedimentary deposits along with gem species such as corundum, chrysoberyl, spinel, garnet, tourmaline, topaz, and zircon (C.B. Dissanayake and M.S. Rupasinghe, “Application of geochemistry to exploration for gem deposits, Sri Lanka,” Journal of Gemmology, Vol. 23, No. 3, 1992, pp. 165–175). Though rarely used as a faceted gemstone, gadolinite is of interest to collectors (E.A. King Jr., “Texas Gemstones,” Bureau of Economic Geology, Report of Investigations No. 42, University of Texas at Austin). To our knowledge, this is the first time gadolinite has been submitted to GIA.
