SAPPHIRE - A CRYSTAL WITH MANY FACETS

To the layperson, a sapphire is a velvety-blue faceted gem. However, sapphire, the single-crystal form of aluminum oxide (also known as corundum), is a mineral and material with significant history and prominence for geologists, gemologists, and materials scientists alike. The formation of corundum in nature is well documented – it is found in igneous rocks, metamorphic rocks, and alluvial deposits. As a primary oxide possessing excellent mechanical and chemical properties, it is extremely stable resulting in extensive deposits located all over the world. Add in desirable optical features including the ability to assume a rainbow array of colors (due to trace impurities), and we have one of the most significant gem varieties of all time. However, the interest does not stop here. Since the beginning of the industrial revolution, corundum has found its way into technological applications as its mentioned exceptional mechanical, chemical, and optical characteristics give it high utility. As a primary oxide with a congruent melting point, growing this crystal synthetically proved to be straightforward, and all of the general bulk crystal growth methodologies developed to date have been applied to sapphire. Today, extraordinarily large single crystals of high purity sapphire can be produced commercially – crystals exceeding 100 kg have been grown! For geologists, gemologists, and materials scientists, to best understand this important crystalline material, recognizing the research that has been done by all of the groups on the mechanisms of growth including the introduction of impurities and defects is instrumental. This talk will present the synthetic crystal growth perspective with the goal of providing the geology community information that can be related back to natural mineral formation conditions - an introduction to the various techniques used to grow sapphire along with how defects and impurities are controlled and characterized.