‘Cobalt-Blue’ Gem Spinels

The traditional gemologicml technique for distin-gr~ishing natural from syntheric blue spinel has involved the use of refractive index and absorption spectra. Recently, however, the standard interpreta-Lion of these tests has proved trorlblesome in the identification of several intensely colored blue stones that have R.Z.'s in the range of natural blue spinels but that exhibit spectral Jeatures generally associated with synthetic cobalt-colored spinels. To establish the origin of rhese specimens, a derailed investigation of natural, flame-fusion, and I ~L I X blne spinels was carried out. Our results indicate that, while spectral feat~lres in the longer wavelengths are unreliable for separating these spinels, an iron-related absorption band at about 460 n m was seen only in our natural specimens. Flame-fusion synthetics m a y still be distinguished b y their lower R.I.'s.