Surface-reaching growth tubes surrounded by intense pink ‘sleeves’ in Mozambique tourmaline is not the result of diffusion treatment, says the Gemological Institute of America (GIA).
In response to the recent appearance of these blue to blue-green, copper-bearing stones, GIA tested several samples. It concluded the pink zones were produced by fluids containing naturally occurring radioactive material.
In all instances where GIA observed pink coloration, growth features surrounded by the pink colour reached the surface of the stones. In those test samples where they didn’t, no pink colour was observed. When viewed down their length, the colour in the pink zones was seen to bleed out into the tourmaline, becoming weaker until gradually fading away. If post-growth matter in the tube created a blockage, coloration occurred only to that point. Cracks extending from or between the growth tubes also showed a pink colour.
According to GIA, radiation is known to produce pink-to-red colour in tourmaline. There have also been reports of smoky quartz and green diamonds with coloration that was caused by exposure to naturally occurring radioactive fluids.
Test samples all originated from Mozambique, suggesting the pink zones are characteristic of that locality.