Gemmology Queensland writes:
Darl cobalt blue spinels from Okkampitiya (Sri Lanka) owe their unusual color to the natural presence of cobalt (up to 5ppm) replacing magnesium. These spinels visually resemble Co-doped synthetic spinels with which they possibly could be confused. However the 1.72 refractive index, 3.60 +/- specific gravity, inertness to both longwave and shortwave ultraviolet light, and the presence of a strong absorption at 458 (460)nm in the visible spectrum the natural cobalt blue spinel readily discriminates it from both its Verneuil and flux-grown look-alike.
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