synthetic corundum.7, hydrothermal, gems

Gübelin investigates two recently introduced gem products—rubies represented as hydrothermally grown and natural beryl coated with synthetic emerald—emphasizing the identification difficulties created when synthetic growth surrounds a natural crystal containing convincing natural inclusions. He first reviews the hydrothermal synthesis of corundum developed by Laudise and Ballman at Bell Telephone Laboratories, describing the transport of dissolved nutrient through a temperature gradient in a pressurized vessel and its deposition on suspended seed crystals. Chromium supplies the ruby colour, while growth conditions and the materials used in the apparatus influence the resulting crystals. The ruby specimens personally examined, claimed to originate from Chatham, consisted predominantly of natural ruby cores covered by relatively thin synthetic layers. Their refractive indices, birefringence, specific gravity, dichroism, absorption spectra, and fluorescence provided no reliable separation from natural ruby. The cores contained rutile silk, mineral inclusions, and liquid feathers, while the overgrowth reproduced the original crystals’ external forms. Identification depended on minute features interpreted as gas bubbles within the surface layer, occurring singly or in lines, clouds, and planes, together with the abrupt termination of natural inclusions at the boundary between core and overgrowth. Gübelin notes that ordinary cutting could remove most or all of these thin coatings. The second product, manufactured by Johann Lechleitner and marketed as Emerita, comprised a previously fashioned natural beryl nucleus overgrown with a thin hydrothermal synthetic emerald mantle. The coating continued the crystallographic orientation of the seed, forming a continuous crystal rather than a cemented assembly. Its thickness varied with crystallographic direction and controlled the apparent colour. Standard gemmological properties again overlapped those of natural emerald, and refractometer readings primarily represented the synthetic surface. Microscopy, particularly with immersion and examination from the side, revealed the dark green rim around a pale or colourless core, occasional reflections at the interface, and characteristic structures within the overgrowth. These included parallel or intersecting tension fractures, partly healed fissures containing residual liquid droplets, and particles provisionally interpreted as other mineral phases, such as phenakite or euclase. Some particles obstructed growth and produced tapering cavities containing liquid and a bubble. High magnification and phase contrast helped clarify these delicate features and their relationship to the growth process. Gübelin concludes that natural inclusions within a seed cannot establish the natural origin of the entire stone. Careful examination must instead distinguish the core from its synthetic mantle. He also stresses that altered manufacturing conditions can eliminate many diagnostic inclusions; in exceptionally clean emerald coatings, the contrast between the green outer layer and pale interior may provide the only decisive evidence.