Lab Notes Gems & Gemology, Spring 2023, Vol. 59, No. 1

Ruby with Interesting Synthetic Overgrowth


A large cavity partially filled with synthetic overgrowth on the pavilion of a heated Burmese ruby in darkfield lighting (left) and reflected lighting (right). Photomicrographs by Nicole Ahline; field of view 7.19 mm.
A large cavity partially filled with synthetic overgrowth on the pavilion of a heated Burmese ruby in darkfield lighting (left) and reflected lighting (right). Photomicrographs by Nicole Ahline; field of view 7.19 mm.

The Carlsbad laboratory received a 3.02 ct ruby for a colored stone identification and origin report. Standard gemological testing was consistent with ruby: a hydrostatic specific gravity of 3.96 and a ruby spectrum in a handheld spectroscope. Internally, the stone displayed white flakes in a hexagonal formation and roiled graining. Partially healed fissures with residue, which result from heating in the presence of a flux, were also documented. The inclusion scene, along with the stone’s trace element chemistry collected using laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS), were consistent with Burmese ruby. This ruby was not treated by beryllium diffusion, as no beryllium was detected by LA-ICP-MS. The curious feature of this stone was a large cavity on the pavilion that was partially filled with synthetic overgrowth that was observed in the microscope (see above).

Synthetic overgrowth can be a byproduct of heating a piece of corundum at high temperatures, with or without the presence of a flux. This creates an environment in the crucible that allows for the partial dissolution of corundum and can result in synthetic corundum overgrowth on the surface and in cavities of the heated stone. Synthetic overgrowth is typically an aggregated structure of hexagonally shaped, platy crystals (Fall 2002 G&G Lab Notes, pp. 255–256). When viewed in reflective lighting, there is no luster difference between the host corundum and the synthetic overgrowth, as the two materials have the same refractive index. The gaps that could be observed between the crystals in this stone showed how they grew in different orientations.

Synthetic overgrowth is common enough that it should always be looked for in corundum that has undergone high-temperature heat treatment. There are cases where synthetic overgrowth can be observed using polarized light, but careful analysis is required because the overgrowth can completely fill cavities and be inconspicuous.

Nicole Ahline is a senior staff gemologist at GIA in Carlsbad, California.