Gemstones need to be distinguished from each other, from synthetic gemstones and from imitations. A synthetic gemstone is chemically and structurally equivalent to its natural counterpart but is made in a laboratory. Synthetic stones can sometimes be very difficult to distinguish from natural ones. Imitations may be any material that bears a superficial resemblance to the original, but are usually glass or plastic. Composite stones like doublets and triplets are a combination of materials, usually a natural one with a backing and/or covering of some other material. Some composite stones should not be submerged in liquid, lest it penetrate between the layers and cause discolouration.
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