Gemstones stones

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... 

Other treatments fall into the three groups (1) summari2ed in Table 5. There are chemical treatments and impregnations which penetrate below the surface of the gemstone. Then there are surface coatings of various types. Finally, there are composite stones. 

Colored Impregnations. Colored oil to fill cracks is used on gemstones primarily to improve color, most frequendy on emerald. Colored oil is also used to simulate other stones, most frequendy quartz. Depending on the dye used, the colors may fade, in addition to the problems associated with colodess oiling. 

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. 

Azurite Hydrous copper carbonate Blue 3.8 3.8 Crystalline Ornamental stone, gemstone, blue pigment, building stone, making lime... 

Cutting, grinding, and shaping stone, and in particular burnishing and polishing the surface of stone as well as metals, requires the use of abrasive materials that are harder than the solids to be cut, ground, burnished, or polished. Sapphire and ruby, two very hard gemstones, for example, can be cut or polished only with the assistance of diamond powder, an abrasive that is harder than sapphire or ruby. Diamond is the hardest material... 

Ruby and Sapphire. Ruby and sapphire are "sister stones" both are gemstone forms of the mineral corundum (composed of aluminum oxide). Pure corundum is colorless, but a variety of trace elements cause corundum to exhibit different colors. Ruby is red corundum, while sapphire is corundum in all colors except red. The red in rubies is caused by trace amounts of chromium the more intense the red color of a ruby, the more chromium it contains. The blue in sapphires is caused by titanium and/or iron impurities (Garland 2002 Hughes 1997). 

The ability to geographically identify the source area of natural gems plays an important role in determining if these stones may have originated from a politically unstable area. Garnet is a semiprecious silicate mineral of variable composition that has been used as a gemstone since the Bronze Age. 

Benitoite is a rare, strongly dichroic, blue mineral used as a gemstone. In spite of much effort in its study, the origin of color in benitoite has not been definitively estabhshed. Because traces of Fe are found, ideas proposed include the Fe -Ti or the Fe -Fe inter-valance charge transfer. While most benitoite is colorless when viewed down the c-axis, there are a very small number of exceedingly rare stones, which are pink in this direction (Rossman 1988). 

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