corundum, Sri Lanka.12, gems

corundum, history.1, India.12, gems, Indian lapidary

beryl, synthetic beryl, emerald, synthetic corundum, gems

Gübelin reviews the diagnostic properties of several recently introduced synthetic gemstones and the problems they pose for separation from their natural counterparts. Emphasis is placed on the combination of conventional gemological properties with microscopic examination of inclusions and growth structures, supplemented where necessary by spectroscopic and other analytical methods. The article discusses characteristic features of newer synthetic materials, particularly synthetic corundum and emerald, and shows that physical constants alone may be insufficient for identification as manufacturing techniques increasingly reproduce the appearance and properties of natural stones. Gübelin stresses the diagnostic importance of flux residues, metallic inclusions, growth zoning and other internal features associated with particular synthesis processes. The paper summarizes practical criteria by which these newer products can be recognized in gemological testing.

corundum, history.1, spinel, gems, Arabic mineralogy, early lapidaries

Vesel examines the historical meaning and usage of the Persian gem terms yāqūt and laʿl, showing that they cannot simply be translated indiscriminately as “ruby.” Through comparison of Persian mineralogical and literary sources, she demonstrates that yāqūt generally denoted corundum and could be qualified by color: red yāqūt corresponding to ruby and blue yāqūt to sapphire, with other color varieties also recognized. Laʿl, by contrast, referred particularly to the gem historically known in Europe as balas ruby, corresponding principally to spinel and strongly associated with the mines of Badakhshan. The article traces the distinctions and occasional confusion between these terms among Persian authors and demonstrates the importance of historical terminology when identifying gemstones described in Islamic and Persian texts. Later scholarship specifically summarizes Vesel’s distinction as yāqūt for true Ceylon ruby and laʿl for the Balas ruby of Badakhshan.

corundum, Burma.12, quality.10, gems

corundum, synthetic corundum.7, gems

Gübelin examines the identification of newly introduced Chatham flux-grown blue and orange synthetic sapphires and of natural sapphires whose color has been altered by heat treatment or surface diffusion. The physical properties of the Chatham synthetics differ little from those of natural sapphire, making microscopic examination particularly important. Blue Chatham sapphires may show irregular blue, yellow, and colorless zoning, straight or angular growth structures, polysynthetic twinning, coarse or delicate flux residues forming feathers, veils, nets, lace-like patterns, and misleading “fingerprints,” together with black platinum platelets, rods, splinters, and needles that reflect with a characteristic silvery metallic lustre; X-ray analysis confirmed the metallic inclusions as platinum. Some blue specimens also fluoresce intense pale green under long-wave ultraviolet radiation. Chatham orange sapphires range from brownish orange through orange-red to pinkish-orange “padparadschah” colors and likewise contain flux residues and platinum inclusions that provide important evidence of synthetic origin; ultraviolet absorption and electron-spin-resonance measurements are also investigated as possible means of separation from natural stones. Gübelin then discusses heat-treated natural sapphires, in which high temperatures may produce pockmarked surfaces, altered or melted mineral inclusions, circular tension fractures with lace-like rims, enhanced cloudy or phantom-like zoning, and fine exsolved particles producing turbidity. Diffusion-treated sapphires may additionally show strongly colored surface-reaching fissures and inclusions, irregular surface coloration, and concentrations of color along facet junctions and the girdle; immersion in a highly refractive liquid makes these features particularly conspicuous. Gübelin emphasizes that no single characteristic is universally diagnostic and recommends careful comparison of macroscopic appearance, physical properties, ultraviolet response, spectra, and especially microscopic inclusions when distinguishing natural, synthetic, heat-treated, and diffusion-treated sapphires.

corundum, synthetic corundum.7, Ramaura, synthetic garnet, gems

Gübelin describes two recently introduced artificial gem materials: Ramaura flux-grown synthetic ruby and yttrium gallium aluminate (YAG), the latter marketed as a green imitation of tsavorite garnet. The first portion discusses the manufacture and gemological properties of Ramaura ruby and emphasizes microscopic features useful for separating it from natural ruby, particularly characteristic flux residues and growth-related inclusions. The later installments treat yttrium gallium aluminate, describing its physical and optical properties and characteristic inclusions. The green material has a high refractive index and specific gravity and contains distinctive reddish-brown flux inclusions and wispy veil-like features. The series illustrates the importance of inclusion microscopy in recognizing artificial gem materials whose appearance may closely resemble natural gemstones. Contemporary abstracts confirm that the second installment concentrates on characteristic inclusions and the third on the physical and optical properties and inclusions of yttrium gallium aluminate.

corundum, color.4, synthetic corundum.7, gems

The Heat Exchanger Method (HEM), a new crystal growth process, is in commercial production for 20 cm diameter sapphire crystals and 40 cm diameter silicon ingots for optical applications. The simplicity of the HEM, combined with a very high degree of control of the submerged, solid–liquid interface, allows growth of high-quality crystals. The HEM is also being adapted for the growth of Co:MgF₂, Ti:Al₂O₃, and Cr:Al₂O₃ crystals for laser applications.

corundum, USA.12, Montana.12, treatments.6, gems

change of color, alexandrite effect, gems

Gübelin discusses the alexandrite effect—the apparent change in color shown by certain gemstones under light sources of different spectral composition. Alexandrite chrysoberyl typically changes from blue-green or green in daylight to red or reddish violet in incandescent light, but comparable behavior also occurs in some garnets, sapphires, spinels, kyanite, fluorite and monazite. The phenomenon is explained in terms of absorption spectra having transmission regions in both the blue-green and red portions of the visible spectrum, separated by strong absorption in the yellow region. Which color predominates therefore depends on the spectral distribution of the illuminating light. The article relates the effect to the chromophoric trace elements present in the various gem minerals and emphasizes that the phenomenon is not restricted to chrysoberyl.