A Remarkable 16.71 ct Transparent Orange-Yellow Sodalite
GIA’s Bangkok laboratory recently examined the 16.71 ct transparent orange-yellow faceted oval shown in the figure above. Standard gemological testing revealed the stone to be singly refractive with a refractive index of 1.484 and a hydrostatic specific gravity of 2.28. The stone showed strong orange fluorescence under long-wave ultraviolet light and weak orange with chalky white fluorescence under short-wave UV. Microscopic examination revealed internal features such as a fingerprint-like plane of two-phase fluid inclusions and numerous tiny transparent crystals. Raman spectroscopy analysis using 514 nm laser excitation showed several distinct peaks at 263, 294, 464, 969, 986, and 1058 cm–1, which are characteristic features of sodalite (B. Lafuente et al., 2015, http://rruff.info/about/downloads/HMC1-30.pdf). Energy-dispersive X-ray fluorescence also identified the main chemical composition as sodium, aluminum, silicon, and chlorine, consistent with sodalite, which falls into the feldspathoid mineral group Na8(Al6Si6O24)Cl2.
Sodalite is typically a semitransparent to opaque blue stone with calcite veining resembling lapis lazuli. However, it can occasionally exhibit other colors. The orange-yellow variety of sodalite is rare, and its color may be related to the presence of sulfide ions (F. Seel, “Sulfur in artwork: Lapis lazuli and ultramarine pigments,” Studies in Inorganic Chemistry, Vol. 5, 1984, pp. 67–89). Ultraviolet/visible spectroscopy showed an absorption band centered at approximately 480 nm, likely caused by heavy radioactive elements creating an unstable color center substituting for Cl– in a tetrahedron of Na+ ions (H. Annerstein and A. Hassib, “Blue sodalite,” Canadian Mineralogist, Vol. 17, 1979, pp. 39–46; P.S. Pizani et al., “Color centers in sodalite,” American Mineralogist, Vol. 70, 1985, pp. 1186–1192). This sodalite is exceptional for its color, transparency, and 16.71 ct size. To the author’s knowledge, this is GIA’s first encounter with a large faceted orange-yellow sodalite.
