Precious gemstone

Turquoise and malachite are semi-precious gemstones made up of copper compounds. Turquoise ranges in color from green to blue. 

A conventional listing of the more important nonmetallics is presented in Table 12.2. Gemstones. A gemstone is a semi-precious or precious natural mineral with exceptional physical properties which, when cut and polished, can be used in jewelry. Only four minerals are considered as precious gemstones sensu stricto diamond, one gem variety of beryl (i.e., emerald green), and the two gem varieties of corundum (i.e., ruby, deep red, and sapphire deep blue). Beside natural minerals synthetic gemstones and their simulants are also found in jewelry. 

A special class of stones are the precious and semiprecious gemstones, with a long history of use in decorations and jewelry. Especially in the Far East and pre-Columbian America, jade was used extensively for carving ceremonial and luxury items. Jade encompasses two different minerals quite similar... 

Composite Gemstones. Many types of composite or assembled gemstones have been made (1). Some are shown in Figure 3. In the United States a doublet has two pieces combined using a colodess cement. If three pieces are used, or if two pieces are assembled using a colored cement, the gemstone is a triplet. The use of composite stones has declined rapidly with the rise of inexpensive synthetics. Frequendy seen are opal doublets, where precious opal is backed by a black material. In opal triplets a thin sHce of precious opal is cemented between a black backing and a clear cover, usually of quartz. Additionally, insects and even fish have been inserted into amber. 

Figure 6.31 The structure of precious opal consists of ordered arrays of silica spheres with spacing of the order of the wavelength of light. These arrays diffract light and give rise to the colours seen in the gemstones...

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