It is difficult to discern the difference between a natural and a synthetic star sapphire. Both types of gemstones have virtually the same physical and chemical characteristics. Each has the sought-after qualities of beauty, colour, strength and hardness
Natural star sapphires originate in the mineral corundum, and display a phenomenon called an “asterism.” Asterisms, caused by tiny, thin, needle-like inclusions or rays (usually six, but sometimes 12) appear to float like stars on the surface of a cabochon cut gemstone. Minuscule particles of foreign matter arranged symmetrically in the natural crystal cause these inclusions. The illusion of the star’s motion depends on the available light source. The more vivid the colour blue, and the more transparent the crystal, the more prized the gemstone.
A French chemist, Auguste Verneuil (1856-1913), perfected the Verneuil process of creating synthetic sapphires through flame fusion in 1892. Then, in 1947, a researcher for the Linde division of Union Carbide accidentally discovered a method for reproducing the asterisms found in natural star sapphires. The Linde innovation led to great commercial success for a few years; however, international competition resulted in diminishing profits and the company ended production of the synthetic gemstones in 1974. Japan currently produces high-quality synthetic star sapphires.
Only a professional gemologist can identify the difference between a natural and a synthetic star sapphire. Most natural gemstones have been heat treated to enhance their colour and clarity. According to geologists at the University of Texas, “Reliably distinguishing among treated, untreated, natural and synthetic ruby and sapphire depends critically on microscopic examination of inclusions and crystal growth features.”