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 whitesapphire and has been called 'Walderite'. Another variety simulates the alexandrite; a green variety has been called 'Amaryl'; and a yellow varietysimulates topaz. The earliest synthetic corundum was the synthetic ruby, nextthe synthetic sapphire. Synthetic corundum stones can be produced as asterias (star stones), to simulate bothstar ruby and star sapphire by the addition of titanium oxide which, at high temperature, is precipitatedas needles of rutile. Such stones can be distinguished from natural stones by use of a microscope which will reveal internal curved striae, and also thepresence 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 morecomplicated.