YAG (yttrium aluminium garnet) is a lab grown material first introduced as a diamond simulant. It was marketed under various names such as, diamonair and diamonique. It is now less used as a diamond simulant, being replaced by cubic zirconia (CZ) which has a much higher dispersion (exceeding that of diamond). It is made in different colours to simulate sapphire, emerald and other gemstones. During manufacture chemical elements are added to determine the colour, for example - chromium for green, cobalt for blue and titanium for yellow. It is a durable stone with good clarity and colour saturation, it also has an exceptional lustre.
YAG is not normally treated.