corundum, synthetic corundum.7, Verneuil, Linde, gems

Gübelin investigates microscopic criteria for distinguishing emeralds from Colombia and Russia’s Ural Mountains, arguing that color alone provides an unreliable indication of origin. Although Colombian emeralds are associated with a velvety green and Russian stones with more yellowish hues, examples from either source may resemble those from the other, particularly in small sizes. A comparative table records their appearances through several colored filters, but the author emphasizes that interpreting these subtle differences requires considerable experience, a sensitive eye, and suitable comparison material. Seeking more objective evidence, he examined numerous emeralds of known provenance, including specimens supplied by the Colombian Consulate in Paris. He identifies minute fluid inclusions, rather than conspicuous fractures or cloudy areas, as the most useful distinguishing features. Colombian emeralds are described as containing scattered, irregular groups of delicate, frequently tail-like cavities. Their boundaries are faint, suggesting that the enclosed liquid has a refractive index relatively close to that of the host. The liquid’s composition was not established, although behavior during heating and considerations of formation led the author to suggest water. The cavities show no consistent crystallographic orientation and remain separate even when grouped. Almost every cavity is reported to contain a tiny cubic crystal, commonly adjoining a rounded gas bubble with a dark outline, so that solid, liquid, and gaseous phases coexist within one inclusion. Carbon dioxide is proposed as the gas, while the solid’s chemical identity remains unresolved. Gübelin regards the cubic form of this solid inclusion as the principal diagnostic feature of Colombian material. Russian emeralds likewise contain multiphase inclusions, but their distribution, cavity shapes, and enclosed crystals differ. Their cavities are more closely associated with fractures and moss-like internal features, lack the characteristic Colombian tails, and contain solid crystals displaying lozenge-shaped or rhombic outlines rather than clearly rectangular cubic forms. The author presents this contrast as a dependable means of separating the two sources and supplies a mnemonic linking Colombian origin with cubic crystals and Russian origin with rhombic crystals. He recommends photomicrography to document the observations and notes that the features can also be examined with a diamondscope. The concluding discussion separates geographical interest from intrinsic quality: identifying a fine emerald as Russian need not diminish its value, while knowledge of provenance may increase an owner’s appreciation of either a Colombian stone or an unusually fine-colored Ural specimen.