Fall 2009

Topaz with Unstable Brown Color

Since early 2007, there has been an influx of orangy to reddish to pinkish brown topaz on the market in Chanthaburi, Thailand (see G. Roskin, “Topaz alert,” JCK, Vol. 178, No. 9, 2007, p. 60). Some of the stones have been represented as coming from Myanmar, others as Brazilian goods. This material has been widely available on the Internet as well. Two of these topazes were supplied to the GIA Laboratory in Bangkok for examination by Jeffery Bergman (LGL Co., Bangkok): an orangy brown sample that had been kept in the dark, and a near-colorless topaz that had faded from orangy brown to almost colorless after being exposed to sunlight for one day (figure 1). The samples were obtained in Chanthaburi, with no disclosure of color instability.

In June 2008, some additional samples of this color-fading topaz (also purchased in Chanthaburi, in November 2007) were brought to GIA’s attention by L. Allen Brown (All That Glitters, Methuen, Massachusetts). To observe the effect of light on this material first-hand, GIA purchased four faceted orangy brown topaz samples (19.18–21.29 ct) from Mr. Brown, and had three of them sawn in half; the fourth stone was retained as a reference sample. One-half of each of the three stones was kept in the dark for comparison, and the other portions were each exposed to different lighting environments: (1) placed on a windowsill in daylight for 80 hours; (2) exposed to a standard 100-watt incandescent light bulb for 80 hours at a distance of 2.5 cm; and (3) exposed to long-wave UV radiation for 40 hours, using a 6-watt bulb at a distance of 2.5 cm. As seen in figure 2, the samples exposed to daylight and incandescent light faded considerably; the fading of the latter piece was probably also enhanced by the heat of the bulb, since much brown topaz loses color above 200°C (K. Nassau, Gemstone Enhancement, 2nd ed., Butterworth-Heinemann, Oxford, U.K., 1994, p. 192). The sample that was exposed to UV radiation faded even further, despite the shorter exposure time.

Some brown topaz (e.g., from the Thomas Range in Utah) may lose its color when exposed to sunlight (M. O’Donoghue, Ed., Gems, 6th ed., Butterworth-Heinemann, Oxford, UK, 2006, pp. 176–177). In addition, O’Donoghue (2006) noted that unstable brown color centers may develop from the laboratory irradiation of colorless topaz. To date there is no gemological test that can identify whether brown color in topaz is due to natural or laboratory irradiation, and a fade test is the only way to determine if the color is stable.

Garry Du Toit (garry.dut@giathai.edu) and
Kamolwan Thirangoon
GIA Laboratory, Bangkok