Chatham, synthetic corundum.7, synthetic ruby, synthetic sapphire, flux, gems

C.R. Burch describes the morphology and diagnostic significance of metallic inclusions in coloured flux-grown Chatham synthetic corundums, emphasising their greater diversity of form than the familiar triangular and hexagonal plates alone would suggest. Detailed microscopic examination covered red, blue and orange-pink material of padparadscha colour, including rough and crystal clusters totalling 1,046 carats, together with twelve faceted stones: five orange-pink sapphires, four blue sapphires and three rubies. Solid flux inclusions occurred in every specimen examined, while many samples also contained well-developed metallic inclusions. These are considered almost certainly platinum derived from the growth crucible, although the author retains the general designation metallic because rhodium or iridium crucibles may also be employed in flux growth and no definitive compositional identification is reported. When appropriately oriented and illuminated, all the metallic inclusions were strongly reflective, appearing silvery, grey or yellowish in reflected light. Triangular and hexagonal outlines were most common, but perfect symmetry was unusual; truncated triangular forms and pseudohexagonal crystals were also illustrated. Other relatively abundant forms resembled thin plates, needles, spicules, daggers, blades, rods, bars and pyramids, with additional inclusions having irregular outlines. Several displayed pitting or etching on one or more surfaces, tentatively attributed to corrosion by molten flux before incorporation into the growing corundum. Some metallic inclusions were partly covered by white flux layers of variable thickness, demonstrating the close association between these two inclusion types. Twelve photomicrographs document examples in ruby, blue sapphire and orange-pink sapphire, using reflected light or combined transmitted and reflected illumination at magnifications of approximately 10–16×. Illustrated features include etched hexagonal crystals, dense groups of differently shaped metallic inclusions, elongated blades with partial flux coatings, a flux-coated needle and a bar with triangular cross-section beneath a faceted stone’s girdle. The author concludes that careful assessment of metallic inclusion shapes, considered together with flux inclusions, can provide valuable evidence of synthetic origin in doubtful corundum. Although the investigation concerns Chatham products exclusively, comparable morphological diversity is suggested as a possibility in other gemstones grown from flux melts.