corundum, synthetic corundum.7, synthetic spinel, inclusions.5, gems

Gübelin describes unusual gas inclusions in synthetic corundum and spinel that closely resemble fluid inclusions in natural gems, emphasizing the need to examine their contents and optical behavior rather than identify origin from shape alone. Natural and synthetic corundum have closely similar chemical and physical properties, while differences in refractive index and luminescence may be too small or overlapping to provide decisive evidence. Microscopic characteristics therefore assume particular importance. The author reviews natural corundum’s oriented rutile needles, other mineral inclusions, fluid cavities, and growth structures, contrasting these with the curved striae and rounded gas bubbles commonly associated with synthetic material. He cautions that twinning occasionally occurs in synthetics and that internal cracks near facet junctions can result from strain in either natural or synthetic stones. Recent examinations revealed rare, irregular gas cavities, particularly in blue synthetic sapphires, with elongated, tubular, wormlike, dendritic, and mesh-like forms. Some resemble coarse liquid feathers in natural corundum so closely that they could lead to misidentification. Small accompanying spherical bubbles and, in one illustrated specimen, curved striae exposed by closing the microscope’s iris diaphragm provide additional evidence of synthetic origin. Dark, hazy areas in another sapphire are attributed to localized concentrations of coloring material. Gübelin distinguishes the illustrated gas-filled cavities from natural liquid inclusions by rotating the stone in an immersion cell: the gas inclusions remain black, whereas liquid-filled cavities appear transparent when viewed through their flat sides and may contain a separate gas bubble. He notes that such deceptive forms were uncommon at the time but might become more frequent as manufacturing improved. The discussion then turns to synthetic spinel, whose variable excess of aluminum oxide affects refractive index and specific gravity, limiting reliance on these properties alone. Although curved striae are absent, occasional gas inclusions may likewise be elongated, wormlike, sharply outlined, or apparently oriented. Anomalous double refraction caused by internal strain offers a further means of recognition. Under polarized examination, synthetic spinel displays incomplete extinction and irregular bright and dark patterns, including cloudy, fibrous, feathery, and crosshatched appearances. The author contrasts these with the weaker, indistinct, blotchy anomalous double refraction sometimes observed in natural spinel, commonly associated with solid or fluid inclusions. He also distinguishes the latter from the more checkered patterns seen in garnet. The article concludes that characteristic strain patterns, considered together with inclusion morphology, cavity contents, and other microscopic evidence, permit recognition of synthetic material despite its potentially deceptive resemblance to natural gems.