Lab Notes Gems & Gemology, Summer 2014, Vol. 50, No. 2

Flux-Grown Synthetic Ruby with Hydrothermal Synthetic Seed Crystal

Zig-zag growth in synthetic ruby
Figure 1. The zigzag-like growth structure observed in this 1.18 ct synthetic ruby is characteristic of hydrothermal growth. Photomicrograph by Nathan Renfro; field of view 1.42 mm.
The Carlsbad laboratory recently received a 1.18 ct transparent red octagonal step-cut stone for ruby report service. Standard gemological testing established the following properties: RI—1.762 to 1.770; birefringence—0.008; optic sign—uniaxial negative; pleochroism—orangy red to purplish red; specific gravity—4.01; fluorescence reaction—strong red to long-wave, weak red to short-wave UV radiation. Examination with a desk-model spectroscope revealed a typical ruby spectrum. All of these properties were consistent with natural or synthetic ruby.

Under magnification, the most distinctive internal characteristic in the crown was the presence of strong irregular growth features: zigzag- or mosaic-like striated patterns (figure 1), typical of a hydrothermal synthetic. Other areas of the ruby lacking these irregular growth features contained hexagonal metallic platelets and high-relief, whitish flux inclusions (figure 2), typical of a flux-grown synthetic. Flux and hydrothermal inclusions have not been previously documented in the same specimen.

Synthetic ruby under fiber-optic lighting
Figure 2. When the synthetic ruby was examined under diffused fiber-optic lighting and darkfield illumination, hexagonal platinum platelets and trapped flux residue became apparent. Photomicrograph by Nathan Renfro; field of view 1.42 mm.
Laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) analysis revealed traces of Ca, Ti, Cr, Fe, Mo, Rh, Sn, W, and Pt. The low amount of Fe and Ti, the absence of V and Ga, and the presence of Pt were consistent with flux-grown corundum.

Both natural and flame-fusion synthetic ruby have been used as seed crystals in the flux growth of ruby (J.I. Koivula, “Induced fingerprints,” Winter 1983 G&G, pp. 220–227; Summer 1991 Lab Notes, p. 112). The seed crystals are generally removed during the cutting process but may, on rare occasions, be detected in finished specimens. Upon close microscopic exami­nation, we noted that several of the flux-filled healed fractures (wispy veils) extended into the areas showing hydrothermal graining. These obser­vations led us to conclude that the hydrothermal material was a seed crystal and that the flux healing was a secondary process to the hydrothermal growth. There was an irregular separation between the materials under brightfield illumination.

This unusual combination of a hydrothermal ruby seed with flux ruby overgrowth is the first of its kind examined by GIA.

Ziyin Sun is a staff gemologist, and Dino DeGhionno is senior manager of colored stones, at GIA's Carlsbad laboratory.