A synthetic gemstone resembling a natural sapphire, first produced by the Verneuil Furnace in 1910. Earlyexperiments had used cobalt oxide, but the colour concentrated in patches, so magnesium oxide was added as a flux; this resulted in a stone that was asynthetic spinel rather than a synthetic sapphire. Later magnetic oxide of iron and titanic acid were added, and a clear transparent bluesynthetic sapphire was produced. The synthetic star sapphire was produced by adding more titanium oxide, which precipitated to form needles ofrutile. A colourless variety (misleadingly called walderite) has been produced by use of pure alumina, and various coloured varieties by use of differentmetallic oxides, e.g. with nickel oxide a pinkish-orange stone and with vanadium oxide a greenish stone resembling alexandrite. The synthetic varietiescan be distinguished by their showing, when viewed through a microscope, curved colour bands as opposed to straight bands, and sometimes minute gas bubbles.