corundum, synthetic corundum.7, Verneuil, asterism, gems

Breebaart compares the microscopic structures and inclusions of American and German synthetic star rubies and sapphires, identifying differences that may help distinguish their manufacturing origins. He first considers stones produced by the Linde Air Company, describing their manufacture by the Verneuil process with titanium oxide added to the starting materials, followed by annealing to produce the oriented needles responsible for asterism. These stones retain features associated with conventional flame-fusion synthetic corundum, but their appearance and distribution differ in several respects. The rutile needles are very fine, short, and densely packed, although occasional coarser crystals, tentatively identified as rutile, also occur. Small spherical gas bubbles commonly form clouds of pinpoint inclusions, generally less densely concentrated than those observed in ordinary synthetic corundum. Curved growth striae provide another characteristic feature, particularly in synthetic star ruby, where their comparatively wide separation makes them more conspicuous than the fine striations of conventional synthetic ruby. Turning to German products then entering the market in increasing quantities, Breebaart notes that their production method is unknown to him, but infers procedural differences from their contrasting internal features. Their oriented rutile needles are longer and coarser than those in the American material. In German star sapphires, pinpoint bubbles occur throughout the stones and appear to radiate from the centre toward the girdle. German star rubies contain bubbles ranging from minute points to elongated, tadpole-like forms, arranged in a circular pattern. Both varieties display widely separated needles, probably rutile, near the girdle, intersecting at angles consistent with corundum’s crystallographic symmetry. Changes of microscopic focus reveal that different inclusions occupy separate layers: rutile needles may be brought into focus before deeper bubble concentrations become visible. The author also describes a transparent circular region surrounded by bubbles and extending approximately halfway through the stone to the needle-bearing zone. Rather than the curved striae characteristic of the American material, the German specimens show complete circular growth bands arranged concentrically. Breebaart proposes a round dark spot surrounded by such bands as an indication of German manufacture. Four photomicrographs illustrate bubble distributions, growth structures, and needle inclusions, with additional straight features tentatively interpreted as a twinning plane or internal fractures. The paper presents these observations as practical microscopic distinctions between the synthetic star-corundum products examined, while retaining uncertainty concerning the German manufacturing process and the identity of some internal features.