Lab Notes Gems & Gemology, Winter 2020, Vol. 56, No. 4

Unusual Absorption in a Blue Flux-Grown Synthetic Sapphire

Blue flux-grown synthetic sapphire.
Figure 1. A 0.52 ct blue flux-grown synthetic sapphire. Photo by Nuttapol Kitdee.

The GIA laboratory in Bangkok analyzed a 0.52 ct blue flux-grown synthetic sapphire (figure 1). Standard gemological properties were typical for corundum. Observation in the microscope revealed various forms of flux or wispy fingerprints and platinum inclusions in a variety of shapes (figure 2). All of these inclusions are typical for Chatham synthetic sapphire (R. Kane, “The gemological properties of Chatham flux-grown synthetic orange sapphire and synthetic blue sapphire,” Fall 1982 G&G, pp. 140–153).

Platinum inclusions of various shapes.
Figure 2. Platinum inclusions in a variety of shapes. Photomicrograph by Charuwan Khowpong.

However, the most interesting feature of this stone was its UV-Vis-NIR spectrum (figure 3). The spectrum had the usual 377, 388, and 450 nm peaks related to Fe3+, as well as the Fe2+-Ti4+ broad band around 560 nm that is responsible for the blue color, but additionally featured an unexpected broad band around 800 nm. The broad band at 800 nm is typical for basalt-related, natural blue sapphire and can also be found in some heated metamorphic blue sapphire (Summer 2019 G&G Micro-World, pp. 264–265). However, it has never been reported in blue lab-grown sapphire, either flame-fusion or flux-grown.

Oriented UV-Vis-NIR spectra of the faceted synthetic sapphire.
Figure 3. UV-Vis-NIR spectra of faceted blue flux-grown synthetic sapphire. Estimated path length 2.51 mm.

We analyzed the trace element composition on the blue flux-grown synthetic sapphire using LA-ICP-MS. The results showed very low amounts of vanadium (V) and gallium (Ga), a combination that is a classic indicator of lab-grown corundum. Other trace elements such as titanium and iron were found at levels on average of about 95 ppma and 1299 ppma. Beryllium was also detected at 12 ppma (table 1), a trace element whose presence is unpredictable in laboratory-grown sapphire. The presence of rhodium (Rh) and platinum (Pt) is an indicator of flux-grown corundum, representing traces of the crucible in which the sapphire was grown.

Sudarat Saeseaw is senior manager of colored stone identification at GIA in Bangkok.