Treatment of Tourmaline Gems

Tourmaline is famous for the wide variety of colors that it occurs in – it is found in virtually every shade of the rainbow. Because of its flexible borosilicate crystal structure, it can accommodate a variety of both color center imperfections as well as color causing transition metals, and minor differences in make up can greatly affect the color. While most tourmaline gems in the jewelry trade are not treated, the percentage of these gems being treated is increasing, especially among the pink and red toned stones. The processes used include both heat and irradiation. Because of the wide variation in chemical composition, treatment methods do not always work the same on stones of the same color from different mines in different regions.

Irradiation generally has two major effects it can develop or intensify either red or yellow color.  Depending on where the stone was mined, and what color it was to begin with, will determine how it reacts to the radiation treatment.  In general very pale pink green or blue darlings who frequently become a deeper pink to read on a radiation; medium blue or green stones may turn a deep purple; yellow stones may become an orange or peach color.  However some deeper green stones actually turned gray, as the red and green colors balance each other.   This red coloration is generally stable to about 400°C.  The radiation treatments are generally done with high energy electrons (sometimes called beta particles). Care must be taken during irradiation to prevent excessive heat as the stones may be subject to cracking.

Heating removes the pink or red component of the coloration producing colorless stones from tanks and Reds blues from some browns and purples and yellows from some orange tones.  While the temperatures at which the coloration disappears varies, virtually all such stones they colorize by about 700° centigrade. It is long been known that some blue green and green term links can be significantly lightened by heating about 650°C.  Depending on the oxidizing or reducing nature of the heat treatment used a variety of results have been reported.  Great care must be taken in heat treating tourmaline both to avoid temperature shock and to make sure the treatments do not exceed 700° centigrade.  At a temperature of about 725°C tourmaline loses essential water of crystallization, the crystal becomes milky and destroyed.