Punsiri Heat Treatment on Basalt-Related Blue Sapphire
Heat treatment is often applied to sapphire to modify color or improve clarity. One known heat-treatment technique is the Punsiri method, typically applied to metamorphic or low-Fe blue sapphires. This treatment lightens the blue color by heating the stones in an oxidizing atmosphere to diffuse holes and aluminum vacancies into stones with low Ti levels (J.L. Emmett, lectures on corundum at GIA Bangkok, Aug. 28 and Dec. 4, 2010). The main characteristic of Punsiri heat treatment is distinct color zoning observable when the stone is immersed in methylene iodide. The stone will display a pale blue to near-colorless layer corresponding to the girdle outline that surrounds a deep blue core.
Recently, GIA’s Bangkok laboratory examined a 1.63 ct blue sapphire, identified by standard gemological testing (refractive index of 1.760–1.769 and an inert reaction under long- and short-wave UV radiation). Microscopic observation revealed only a few inclusions, consisting of stringers and small healed fractures. We then immersed the stone in methylene iodide and observed Punsiri-type color zoning (see above). Fourier- transform infrared (FTIR) spectroscopy is a common technique used in advanced gemological laboratories to determine whether a stone is heated or unheated. When the FTIR spectrum shows Punsiri features (e.g., multiband structures at 3191, 3064, 2626, and 2412 cm–1; see chapter 4 of R.W. Hughes, Ruby and Sapphire: A Gemologist’s Guide, RWH Publishing, Bangkok, 2017), this is conclusive for heat-treated metamorphic blue sapphire. However, when the FTIR spectrum does not show these features, it does not necessarily mean the stone has not been heat treated. Such was the case for this stone, which showed signs of heat treatment through its distinctive color zoning and near-colorless rim.
UV-visible near-infrared (UV-Vis-NIR) spectroscopy revealed a broadband at about 880 nm, indicating a blue sapphire of basalt-related origin. Laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) showed no beryllium and limited amounts of other trace elements: 7 ppma Ti, 860 ppma Fe, and 47 ppma Ga were detected. This is the first time we have observed Punsiri heat treatment on a basalt-related blue sapphire. It was possible to perform the treatment on this sample due to the low amount of Ti, which allowed for faster diffusion. This report demonstrates the importance of the immersion technique in identifying heat treatment.
