Gold jewellery can be made with some special (unusual) colour effects, either due to intrinsically coloured intermetallic phases or to surface layers grown by reaction with alloying metals or by deposition techniques. A wide range of colours such as blue, black, brown and purple can be achieved and a number of techniques employed. However, there are some drawbacks: intermetallic colours are intrinsically brittle and surface coatings, by any technique, are liable to be fragile and will spall if knocked, and wear away if rubbed constantly. In simple terms, these are analogous to chemical compounds. Gold jewellery with such colour effects is commercially available and becoming increasingly popular. Some of these exotic colours are obtained by alloying gold with certain other metals to produce special alloy structures known as intermetallic compounds. which possess the desired colour. Solid pieces of coloured gold intermetallics can be made by vacuum melting gold and aluminium in the correct ratio and casting. These are brittle and cannot be worked in the traditional manner. However, they can be applied to a substrate, such as a conventional carat gold, by thermal spraying in a gas jet.
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