as well as EVERY SATURDAY in AUGUST
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A synthetic gemstone that resembles the natural spinel. It was first produced accidentally in experiments to produce a synthetic sapphire by the Verneuil Furnace process and using cobalt oxide and also magnesium oxide as a flux. It can be made now to imitate gemstones of several other species, e.g. ruby, sapphire, diamond, zircon, tourmaline, aquamarine, alexandrite, etc. The synthetic stones can generally be distinguished by lack of curved growth lines (striae), by colour banding, and by crystal inclusion, and also by occasional internal globular gas bubbles, as well as by slightly different colour, refractive index, and specific gravity. Other varieties of synthetic spinel include an imitation of lapis lazuli (produced by sintering powdered colourless synthetic spinel and cobalt oxide) and of moonstone (produced by applying heat treatment to a colourless synthetic spinel). There is a synthetic blue spinel that can be distinguished from natural blue spinel by use of a colour filter, and a synthetic red spinel that resembles ruby but is distinguished by its fluorescence.
From: An Illustrated Dictionary of Jewelry, autor: Harold Newman, publishers: Thames and Hudson