Pink in Tourmalines from Natural Irradiation
By JCK Online Staff -- JCK Online, 1/28/2009 3:46:00 PM
The Gemological Institute of America Laboratory has determined that the pink zones in blue to blue-green, copper-bearing tourmaline gemstones were produced by fluids containing naturally occurring radioactive material.
The GIA lab members, responding to concerns in the trade over samples of these particular tourmalines with surface-reaching growth tubes surrounded by intense pink "sleeves," investigated several of these tourmalines in the past year.
In all instances where pink coloration was observed, the growth features surrounded by the pink color reached the surface of the stones, according to the GIA lab team led by chief gemologist John I. Koivula. In cases where growth tubes did not reach the surface, no pink color was seen. When these pink zones were viewed down their length, the color was observed to bleed out into the surrounding tourmaline host, becoming weaker until it gradually faded away. If post-growth matter in the tube created a blockage, coloration occurred only to that point. In addition, any cracks extending from or between the growth tubes also showed a pink color.
Radiation is known to produce pink-to-red color in tourmaline. The coloration of surface-reaching features in tourmaline by invading radioactive fluids has not been reported in the literature; however, there have been reports of both smoky quartz and green diamonds with coloration that was caused by exposure to naturally occurring radioactive fluids. This mechanism explains all the observations of pink and red in these tourmalines.
"Since radiation is the cause of pink color in tourmaline, the presence of these features should not be attributed to any type of intentional diffusion, but rather to the influx of radioactive fluids in their post-growth environment," Koivula said.
All the copper-bearing tourmaline samples with this feature observed thus far have come from Mozambique. This suggests that this type of inclusion feature may be characteristic of that locality.
The presence of the pink zones in these otherwise blue to blue-green gems also provides proof that the host tourmalines were not heat treated, since the temperature needed to treat copper-bearing Mozambique material exceeds the published stable temperature for pink-to-red color in tourmaline, the lab team said.
Other members of the lab team involved in the investigation are: Kevin Nagle, Andy Shen and Philip Owens.
Captions:
Top: This 27.63 ct. copper-bearing tourmaline (examined for this report) contained a large, obvious pink growth tube. Photo by Robert Weldon.
Middle: When these tubes are viewed near-parallel to their length, a pink zone can be observed immediately surrounding the tube. The growth tube is 0.06 mm in diameter. Photomicrograph by John I. Koivula.
Bottom: As shown here, cracks extending from and between the tubes are also often lined with a pink zone as well. Approximate horizontal field of view is 2.15 mm. Photomicrograph by John I. Koivula.
