Acta Petrologica et Mineralogica

The natural Tanzanian zoisite (tanzanite), with its distinctive blue-violet tone, is a rare and valuable gemstone. Heat treatment can optimise the color of zoisite and make it more economically valuable. However, the exact cause of this captivating blue-violet of zoisite has been a subject of debate for years. This study presents an examination of natural yellow and blue zoisites. The color changes of stones at different thermal stages were observed by setting three different temperature conditions 300℃, 400℃ and 500℃, and using the L^*a^*b^* color parameter. The research findings indicate that the gemological properties of different colored zoisites remain consistent before heat treatment. It was determined that a temperature of approximately 500℃ is optimal for achieving a light blue-violet tone in zoisite. Heat treatment results in a pronounced enhancement of the blue-violet hues, enabling the modulation of zoisites’ yellow or brown undertones through temperature control. Notably, the yellow zoisite experiences a fascinating color transformation from yellow-brown to blue-violet, with the critical temperature range between 400℃ and 500℃. Using energy dispersive X-Ray spectroscopy (EDXRF), laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) and ultraviolet and visible spectrophotometry (UV-VIS) techniques, the study identified that the blue color in natural zoisite is primarily caused by V³⁺ and Ti³⁺ ions working in tandem. On the other hand, the yellow color is mainly attributed to Ti³⁺. The V/Ti ratio plays a supporting role in coloration of natural zoisite. Moreover, the development and intensification of blue-violet tones in heat-treated zoisite, as well as the transition from yellow to blue-violet, are intricately linked to the charge migration of Ti³⁺+V⁴⁺→Ti⁴⁺+V³⁺.

Tanzanian zoisite; heat treatment; color; the cause of coloration