A synthetic gemstone resembling the natural emerald, produced by several processes. Early experiments by P.C. Hautefeuille and A. Perrey, c. 1890, producedstones too small to be cut as gemstones, and later attempts by use of the Verneuil Furnace failed because thesubstance fused as glass rather than in a crystalline form. The first success came in 1928 by use of the hydrothermal process by E.Nacken, of Frankfurt. Then in 1934 a synthetic emerald of commercial size, called Igmerald, was produced in Germany by the I.G. Farbenindustrie, using the flux fusion process, followed in 1935 byC.F. Chatham in San Francisco, in 1963 by W. Zerfass in Germany and by Pierre Gilson in France, and in 1965 by the Linde Co. of California. These and other synthetic types later produced can bedistinguished from natural stones by their being lighter and having lower refractive index and specific gravity, as well as by the use of a colour filter; but more effective tests are by microscopicexamination which reveals in the synthetic stones the character of the feathers (two-phase inclusions, rather than the three-phase inclusions of a true emerald) and by tests for fluorescence which isstrong in the synthetic stones.