COLOR SHIFT AND PLEOCHROISM IN TANZANITE

Colored stones are increasingly accepted in China gem market in recent years. Tanzanite owing to its unique violetish blue color is readily recognized and attracts broad attention. Natural, unheated blue tanzanite is known for its trichroic colors in bluish violet, violetish blue, yellow-green. Much lesser known, tanzanite also exhibits metachromatism, a color shift, when illuminated by cold and warm light sources.

However, limited existing reports and studies have been mostly descriptive on general gem deposit and basic gemological characteristics. Very little data exit on color shift and pleochroism distinctive to tanzanite. Quantitative analysis on these color characteristics under heating virtually is lacking, even though over 95% tanzanite materials on gem market are reported to have been heated to improve its appearance.

This study presents direct observational results on color shift with controlled warm (`3200K) and cold (`6500K) light sources under polarization and known crystallographic orientations. Further quantitative analysis have been performed by polarized spectroscopic measurement on trichroic to dichroic color conversion in tanzanite samples from a set of heating experiments between 650 and 900 °C. Optimal heating temperature for these experiments was determined by thermal analysis (TG-DTA), which showed that tanzanite became unstable above 960 °C.

Our optical observation and quantitative results revealed that a yellow-green coloration along the c-crystallographic direction converted and virtually merged into existing blue coloration after heating. Consequently, tanzanite converted from natural, unheated trichroic to heated dichroic.