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Cut gemstones possess all the physical properties of the mineral from which they are cut, although the crystal shape is no longer evident.

If a mineral is very hard, like sapphire, the cut gem will be very durable and will scratch softer stones like quartz. If a mineral cleaves (or splits) easily upon impact, like topaz, the cut gemstone will also have this capacity, and need protection against knocks. If the mineral possesses a high degree of dispersion (the ability to split white light into its spectral colours), the gemstone will be full of 'fire', like diamond. In sphene, the gem variety of the mineral titanite, dispersion is even more pronounced.

Accordingly, gemstones are identified by the same scientific methods as minerals. However, since they are beautiful and valuable, no destructive tests that leave a mark on the stone are used. A range of scientific instruments has been devised especially for testing gemstones. People who are qualified to identify gemstones using these instruments are called gemmologists.

Gemstone lookalikes

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.