Heat treatment of rubies and sapphires at high temperatures sometimes leaves redeposition caused by the partial dissolution of the corundum in the crucible; this is referred to as synthetic overgrowth (Fall 2002 GNI, pp. 255–256). GIA’s Tokyo laboratory recently had the opportunity to test a red oval mixed cut weighing 2.00 ct and an orange round mixed cut weighing 0.89 ct, both of which showed signs of synthetic overgrowth after testing.
Standard gemological testing and advanced testing including chemical analyses revealed that the red oval was a heated ruby of Burmese origin with moderate flux residues in fissures. The orange round was identified as a beryllium-diffused sapphire. Both stones had synthetic overgrowth around the girdles and/or cavities (figures 1 and 2) that could be confirmed by different reactions under cross-polarized light. We tested the overgrowths with laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) with calibration using NIST 612 and 610 standards.
The chemical compositions of these synthetic overgrowths were different from those of corundum; they also varied between the samples. Sodium (9980 ppmw), calcium (40,100 ppmw), magnesium (84,400 ppmw), and silicon (23,400 ppmw) were extremely high in the overgrowth on the flux-heated ruby; beryllium (36,650 ppmw) was extremely high and silicon (909 ppmw) very high in the overgrowth on the beryllium-diffused sapphire (table 1). Boron was detected only in the overgrowth on ruby (59 ppmw). Aluminum was not quantified since it was used as an internal standard. However, the high aluminum signal indicated that the overgrowth contained corundum and/or alumina-related material.
|TABLE 1. Elements in heated ruby and Be-diffused sapphire and their synthetic overgrowths detected by LA-ICP-MS (avg. concentration in ppmwa).|
|Heated ruby||Be-diffused sapphire|
a All detection limits are below 4 ppmw except for Si (heated ruby: 113.5 for stone, 104.1 for overgrowth; Be-diffused sapphire: 93.3 for stone, 80.0 for overgrowth) and Ca (heated ruby: 16.95 for stone, 15.79 for overgrowth; Be-diffused sapphire: 16.95 for stone, 15.79 for overgrowth).
b bdl: below detection limit
c These values were linearly extrapolated with pulse-analog linear dynamic range measurement of the instrument.
Flux is used for artificial heat treatment of rubies, and there are many types of fluxes such as oxides, borates, silicates, molybdates, and fluorides, as well as various combinations of materials (J.L. Emmett, “Fluxes and the heat treatment of ruby and sapphire,” Fall 1999 G&G, pp. 90–92). During flux-assisted heat treatment with or without diffusion, material from the surface of the corundum as well as the alumina crucible and inclusions will dissolve to some degree into the molten flux. On cooling, the flux becomes supersaturated in alumina and crystallizes out onto the nearest convenient surface, often on the stones themselves (J.L. Emmett et al., “Beryllium diffusion of ruby and sapphire,” Summer 2003 G&G, pp. 84–135). Uncommon elements for corundum such as sodium, calcium, and silicon obtained from the overgrowth on heated ruby in high concentration indicate that the overgrowth was mainly derived from fluxes. These elements are comparatively insignificant for the overgrowth on beryllium-diffused sapphire. On the contrary, extraordinarily high beryllium in the overgrowth on Be-diffused sapphire indicates deposition of this additional element in the treatment. Other elements, such as magnesium and zirconium (detected at moderate to high levels), and metals such as nickel, copper, zinc, and lead (detected from both overgrowths at lower levels) may be from the inclusions or the flux.
LA-ICP-MS results (table 1) also indicate that the ruby could be misidentified as Be-diffused since the overgrowth contains a certain amount of beryllium, a possible by-product of the furnace. It is important for gemologists to check other elements such as magnesium, silicon, and calcium in order to confirm that LA-ICP-MS was performed on the overgrowth.