corundum, synthetic corundum.7, gems

Verneuil investigates the causes of the blue coloration of sapphire, prompted by his successful reproduction of “oriental sapphire” by fusing alumina with titanic acid and iron oxide. Earlier analyses had generally attributed sapphire’s color to iron oxide and had not detected titanium in clear, inclusion-free material. Verneuil therefore analyzed carefully selected natural blue sapphires from Australia, Burma, and Montana, finding iron oxide together with small but consistent quantities of titanium: Australian sapphire contained 0.92% iron oxide and 0.031% titanic acid; Burmese sapphire 0.72% and 0.04%; and Montana sapphire 0.56% and 0.058%, respectively. Chromium oxide was absent, while silica occurred only in traces and phosphoric acid was absent from the Australian and Montana stones. From the close similarity of these results in sapphires from widely separated localities, and from the fact that his synthetic sapphire reproduced the natural blue using the same elements, Verneuil concludes that the color results from the combined presence of iron oxide and a titanium oxide. He leaves unresolved whether the principal coloring agent is blue titanium oxide, present in extremely small quantity, or a compound of titanium and ferrous iron, noting that further synthetic experiments might resolve the question.