Feature Gems & Gemology, Spring 1999, Volume 35, No. 1

The Separation of Natural from Synthetic Colorless Sapphire

Greater amounts of colorless sapphire—promoted primarily as diamond substitutes, but also as natural gemstones—have been seen in the gem market during the past decade. In the absence of inclusions or readily identifiable growth structures, natural colorless sapphires can be separated from their synthetic counterparts by their trace-element composition and short-wave ultraviolet (SWUV) transparency. Energy-dispersive X-ray fluorescence (EDXRF) analysis shows higher concentrations of trace elements (i.e., Fe, Ti, Ca, and Ga) in natural sapphires. These impurities cause a reduction in SWUV transparency that can be detected by UV-visible spectrophotometry (i.e., a total absorption in the UV region below 280–300 nm, which is not seen in their synthetic counterparts). This article describes a SWUV transparency tester that can rapidly identify parcels of colorless sapphires.