GIA’s journal Gems & Gemology recently published an interesting revelation: two stones submitted to its New York lab for identification were found to be “unusual natural sapphires” coated with an overgrowth of synthetic ruby.

The article, written by Tyler Smith, a staff gemologist at GIA in New York; and Hollie McBride a staff gemologist at GIA in Carlsbad, California, said that  standard gemmological testing on the two red oval mixed cuts showed refractive indices slightly higher than expected for corundum.

“To the unaided eye, the stones appeared to be chromium diffused, displaying red colour concentration at the facet junctions, but their synthetic component became apparent upon further observation,” the writers noted.

“Microscopic examination revealed that planes of minute particles demarcated a thin layer, approximately 0.3 mm thick, of synthetic overgrowth from a natural sapphire core,” the writers explained. “The presence of intact silk and heat-altered crystal inclusions confirmed the seeds to be natural. A series of shallow flux-fingerprints cracking the surface suggested a flux growth process. Portions of the near-colourless core could be seen in areas where the overgrowth was cut away during polishing. These windows showed a sharp boundary without “bleeding” of the saturated red overgrowth into the near-colourless seed, which is typically observed in chromium-diffused corundum (McClure et al., 1993). The windows into the near-colourless seed also indicate that most or all of the red colour is concentrated in the synthetic ruby overgrowth.”

The article states that the “overall appearance” of the two stones was akin to “the now-rare Lechleitner synthetic overgrowth emeralds, which are composed of a near-colourless beryl seed with synthetic emerald overgrowth”.

As it happened, by sheer co-incidence, the Carlsbad laboratory had received a sample showing this type of synthetic emerald overgrowth.

 “In addition to emeralds, Lechleitner also experimented with ruby overgrowth on both synthetic and natural corundum seeds,” the authors noted, quoting sources. 

Advanced testing on the synthetic ruby overgrowth layer using laser ablation revealed the presence of Cr, Fe, Ga, Mn, Mg, Ni, Ti, Pt, and Zr, the authors said. “These results, specifically the presence of platinum, are consistent with previously reported synthetic ruby overgrowth. Molybdenum was reported in the primary examination of Lechleitner overgrowth rubies, and its absence here suggests alternative growth conditions for these samples.”

The authors also stated that though synthetic ruby overgrowth on natural sapphire seeds has been reported earlier, this was the first instance that the New York and Carlsbad laboratories have had them submitted for identification. “The resurfacing of these vintage overgrowth synthetics shows that once a material is in the trade, it is here to stay,” said the authors.

Pic Caption: A fuzzy heat-altered crystal inclusion commonly referred to as a “snowball” is surrounded by a discoid fracture, proving the natural origin of the corundum seed. Photo by Hollie McBride; field of view 1.26 mm.

Pic Courtesy: GIA