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A synthetic gemstone resembling a natural sapphire, first produced by the Verneuil Furnace in 1910. Early experiments 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 a synthetic spinel rather than a synthetic sapphire. Later magnetic oxide of iron and titanic acid were added, and a clear transparent blue synthetic sapphire was produced. The synthetic star sapphire was produced by adding more titanium oxide, which precipitated to form needles of rutile. A colourless variety (misleadingly called walderite) has been produced by use of pure alumina, and various coloured varieties by use of different metallic oxides, e.g. with nickel oxide a pinkish-orange stone and with vanadium oxide a greenish stone resembling alexandrite. The synthetic varieties can be distinguished by their showing, when viewed through a microscope, curved colour bands as opposed to straight bands, and sometimes minute gas bubbles.
From: An Illustrated Dictionary of Jewelry, autor: Harold Newman, publishers: Thames and Hudson