gems, corundum, treatments.6, sapphire, pressure treatment, heat treatment

Sapphires heated with pressure reportedly entered the trade already about 10 years ago (Hughes et al., 2019), although in rather small quantities. Described by several gemological articles in the past few years (Choi et al. 2014a, 2014b, Song et al. 2015, Leelawatanasuk et al. 2016, Choi et al. 2018), this new variation of heat treatment came into the spotlight only recently, based on claims that sapphires heated with pressure have a stability issue (Peretti et al. 2018 and 2019). As a consequence, there have been extensive discussions about this topic among gem laboratories and within the gem trade, resulting in several press releases (e.g. LMHC 2018) and statements (AGTA 2018). The following article is mainly based on a presentation (pdf file of presentation, LMHC 2019) given at the GILC Conference (www.gemstone.org/events/gilc) in Tucson 2019 summarizing the results of a joint research project and extensive discussion between the following laboratories and entities (in alphabetical order): CGL Central Gem Lab, Japan; CISGEM, Italy; German Gem Lab (DSEF), Germany; Dunaigre Consulting, Switzerland; Gemological Institute of America (GIA), USA; Gem and Jewelry Institute of Thailand (GIT); GJEPC-GTL, India; Gübelin Gem Lab, Switzerland; Hanmi Lab, South Korea; ICA GemLab, Thailand; Lotus Gemology, Thailand; Swiss Gemmological Institute (SSEF), Switzerland.