Synthetic Gemstones
Synthetic gemstones are referring to gemstones that are made in laboratories, these gemstones have virtually the same chemical composition and crystal structure as their natural gemstones, and hence their optical and physical properties are very similar. Many gems have been synthesized in the laboratory, but only a few were produced commercially. Some synthetic gemstones are being used as imitations for other gemstones, for instance, synthetic spinel produced by the flame fusion method is used to imitate gems such as ruby, sapphire, aquamarine, blue zircon, tourmaline, peridot and chrysoberyl.
Imitation
Imitation gems have the appearance of their natural counterparts; most imitations only have the colour similar to the genuine stone, and are made to deceive. But their physical properties, such as hardness or dispersion are different and could not be reasonable imitated. Man-made materials, such as glass and synthetic spinel, have been used to imitate many different gems, but natural stones can also be modified to resemble more valuable gems. Porcelain, enamel, and resins as well as other plastics also serve as gemstone imitations.
Assembled Stones
One of the most common imitations are fabrications called assembled stones, these stones are carefully constructed in which one part may be a genuine gem, another part glass, colourless synthetic (synthetic spinel) or genuine stone (quartz) or plastic. There are many combinations; sometimes two natural gemstone pieces are used to cement the piece together. Stones with two parts are called doublets; stones with three parts are called triplets. Garnet-topped doublet is a common assembled. A thin section of natural stone is cemented to a coloured glass base, which gives the doublet it apparent colour. The deception is most easily seen at the junction of the two layers.