Chalcedony is a finely crystallized or fibrous quartz that forms rounded crusts, rinds, or stalactites in volcanic and sedimentary rocks. Chalcedony is a precious stone that occurs in many forms, colors, and shapes. Chalcedony, agates, jaspers, bloodstone, cornelian, onyx, and chrysoprase all make up the chalcedony gemstones. Chalcedonic pseudomorphs after other minerals often give rise to very interesting specimens. 
Other physical properties are those of quartz. 

 

The Romans prized chalcedony as seals, and the Victorians carved them into an endless array of cameos and intaglios. Chalcedony was used in Renaissance magic for health and safety. In all ages chalcedony has been the stone most used by the gem engraver, and many colored varieties are still cut and polished as ornamental stones. 
Chalcedony is one of the gemstones, that used in commesso, also called florentine mosaic. Commesso is a technique of fashioning pictures with thin, cut-to-shape pieces of brightly colored, semiprecious stones, developed in Florence in the late 16th century. The stones most commonly used are agates, quartzes,chalcedonies, jaspers, granites, porphyries, petrified woods, and lapis lazuli. Commesso pictures, used mainly for tabletops and small wall panels, range from emblematic and floral subjects to landscapes. 
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