Physical Properties and Origins of Synthetic and Diffused Star Corundum

This paper analyzes the chemical composition and physical properties of three star corundum specimens based on routine gemmological tests, electronic probe analysis and ultraviolet-visible spectrum analysis. In addition, the authors analyze the origin of star corundum. The observations show that the tested three specimens appear orange-red, brownish red and mauve and have relatively high transparency. Under strong reflected light, these three specimens exhibit cloudy surface features. The star lines from the surface of the stones are diffused and curved. At the intersection of the star lines, a bright light area can be seen. Through a microscope, these three specimens show well-developed pits and mocks. Two specimens show poorly-defined fibrous aluminum titanate inclusions, but one shows melted surface. All of these features indicate that the fibrous aluminum titanate inclusions seen in these three specimens may be produced by diffusion at a high temperature but annealing after being synthesized. All of these three specimens show obvious closed-arranged curved growth striations, indicating an origin of flame fusion. The electronic probe analysis results, absorption spectrum observations and ultraviolet-visible spectrum analysis give a Cr coloring element for these three specimens. Therefore, these three specimens should be named star ruby, and the color is caused by the synthesis but diffusion origin. However, the asterism is caused by the reflection from fibrous aluminum titanate inclusions produced by diffusion.