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A synthetic gemstone that has been made with many colouring agents, producing stones in many hues and shades. A colourless variety, produced with pure alumina free from potash, correspond to white sapphire and has been called 'Walderite'. Another variety simulates the alexandrite; a green variety has been called 'Amaryl'; and a yellow variety simulates topaz. The earliest synthetic corundum was the synthetic ruby, next the synthetic sapphire. Synthetic corundum stones can be produced as asterias (star stones), to simulate both star ruby and star sapphire by the addition of titanium oxide which, at high temperature, is precipitated as needles of rutile. Such stones can be distinguished from natural stones by use of a microscope which will reveal internal curved striae, and also the presence of tiny spherical gas bubbles (although such bubbles may be minimized by modern processes); more exact tests require use of ultraviolet light and X-rays. Synthetic ruby and synthetic sapphire are now produced by other methods, and detection is more complicated.
From: An Illustrated Dictionary of Jewelry, autor: Harold Newman, publishers: Thames and Hudson