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Imitation gems have the appearance of their natural counterparts, but their physical properties are different. They are made to deceive. 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. It is possible to enhance authentic gemstones by hiding cracks and flaws, or by heat-treating or irradiating to improve their colour.

Glass has been used for centuries to imitate gemstones. It can be made either transparent or opaque, in almost any colour, and, like many gems, has a vitreous lustre. At first sight, therefore, it may easily be mistaken for the real thing. However, it can usually be detected by its warmer feel, and by the evidence of wear and tear that results from its greater softness. Chipped facets and internal swirls and bubbles are common. In addition, unlike most of the gems it imitates, glass is singly refractive.

Glass Ruby


Heating may enhance or change the colour or clarity of some gems. Techniques range from throwing gems in a fire to "cook", to use of sophisticated equipment. The outcome is certain for some gems (like aquamarine, changing from green to blue), but less so for others.

Yellow topaz heated to Blue


Stains, dyes, or chemicals can alter the appearance of a gem, coating just the surface, or changing the whole specimen. For staining to be effective, a stone must be porous or contain cracks and flaws through which the colour can enter. Porous white howlite, for example, can be stained imitate turquoise.



Stained howlite


Gems may change colour if exposed to radiation. This may come from radioactive elements within the Earth's crust, or from artificial sources. Natural radiation may take millions of years to have an effect, while artificial irradiation may take only a few hours to change a gem's colour. In some cases a gem will revert to its original colour, or may fade with time. Many changes can be reversed or modified by heat treatment.


Oils may enhance a gem's colour and disguise fissures and blemishes. It is common to oil emeralds in order to fill their natural cracks and flaws.



Oiled emerald

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