synthetic corundum.7, synthetic ruby, hydrothermal, gems

A hydrothermal ruby acquired by the Gemmological Association is described, with emphasis on the microscopic features distinguishing its natural seed from the synthetic ruby grown around it. The specimen had a relatively thick hydrothermal overgrowth deposited on a natural ruby core. Examination by E. Gübelin confirmed observations he had published in 1961 on hydrothermal ruby produced in the United States. Under dark-field illumination, the seed was readily visible and clearly distinguishable from the surrounding synthetic material. Its natural inclusions consisted of microlites surrounded by liquid films, whose appearance led Gübelin to suggest that the seed was probably a ruby from Siam, now Thailand. The geographical attribution was tentative and based on the appearance of the natural inclusions. The hydrothermal overgrowth contained characteristic gaseous markings, illustrated as irregular tubes and hose-like forms. Photomicrographs taken in transmitted light showed these inclusions and their relationship to the natural core. Dark, paired growth lines meeting at angles characterised the natural seed, while numerous gaseous inclusions occurred along the boundary between the seed and its synthetic overgrowth. A further illustration showed the overall distinction between the two components and the natural inclusions retained within the core. The specimen’s physical properties corresponded to those reported by Gübelin in 1961, although numerical measurements were not supplied in this account. Its colour was dark red, an appearance the report considered sufficient to arouse suspicion. Nevertheless, a gemmologist who examined it briefly under a microscope incorrectly concluded that it was natural. This incident demonstrates the identification problem presented by a synthetic overgrowth surrounding a naturally included seed: observing natural inclusions in one part of a stone does not establish that the entire specimen is natural. The report therefore stresses careful microscopic examination before determining a ruby’s nature, particularly recognition of the boundary between the core and overgrowth and the contrasting inclusion populations on either side. Dark-field illumination was especially effective in revealing this distinction.