Hydrothermal Synthetic Ruby
Hydrothermal synthetic ruby was first introduced to the market in the 1960s (A. Peretti and C.P. Smith, “An in-depth look at Russia’s hydrothermal synthetic rubies,” JewelSiam, April-May 1993, pp. 96–102). The hydrothermal process is very slow and requires high heat and very high pressure (400–600°C, 5,000–30,000 psi), mimicking the conditions deep in the earth that result in the formation of natural ruby. In this method, the corundum seed plates are suspended inside the growth chamber and nutrients are placed in the bottom. Slowly the nutrients dissolve and form a solution which deposits new synthetic corundum growth onto the seed crystal. The size of the synthetic crystal depends on the size of the seed, amount of nutrient solution, and time.
The Carlsbad laboratory recently received a 4.55 ct transparent cushion-cut hydrothermal synthetic ruby for an identification report (figure 1). This type of laboratory-grown ruby is seldom submitted to the laboratory today, though they were more prevalent in the 1980s. Gemological testing yielded an RI of 1.761–1.769 and a specific gravity of 3.99. Its fluorescence reaction was strong red to long-wave and medium red to short-wave UV radiation, and its pleochroism was orangy red to purplish red. These properties are typical of most rubies, natural or laboratory grown.
Chemical analysis was done by EDXRF and showed a high amount of chromium (1550 ppma), low iron (153 ppma), titanium (25.5 ppma), no gallium, and very low vanadium (2.24 ppma), which matches the synthetic corundum chemistry.
Magnification showed characteristic internal features of hydrothermal synthetic corundum such as subparallel striations and angular/zigzag (chevron) graining using a standard gemological microscope in conjunction with fiber-optic light or diffused light (figure 2).
The baseline-corrected spectra showed very strong water peaks in the region between 3200 and 3600 cm–1, which is strongly indicative of hydrothermal synthetic origin. (The instrument was baseline corrected to remove the water signal from the atmosphere as well as having the sample chamber purged with dry air to minimize the detection of water from the atmosphere.) Luminescence images in an ultra-short-wave UV radiation DiamondView showed an interesting chevron growth pattern as well with red fluorescence, further confirming hydrothermal synthetic origin (figure 3).
While hydrothermal synthetic rubies are not as common in the gem trade today, it is helpful to review the distinctive features and characteristics that make them identifiable.