This is the name given to the microcrystalline varieties of quartz that form concretionary deposits (partially of organic origin in the case of jasper). They have been used since time immemorial both as gems, because of their color, hardness, and ability to take a good polish. and as precious materials for the production of ornaments or small sculptures.
The different combinations of colors and patterns have given rise to a specialized nomenclature that was once of great importance. The name chalcedony probably comes from Calcedon or Calchedon, an ancient port on the Sea of Marmara, in Asia Minor. Ornamental materials were mined in that area, and it was an active trading center for precious stones of various types and origins. The Greek khalkedon and Latin charchedonia do not appear, at least from the description of Pliny the Elder, to be the same mineral as the modern chalcedony.
Appearance The typical color is blue whitish-gray, but for ornamental purposes, the types that have been variously colored by small quantities of other elements are usuafly used. These colors can cover the entire mass, as with jasper, or just a few thin, successive layers, as with agate and onyx. The most highly prized colors for the concretionary varieties, which are translucent to semiopaque, are brownish yellow (sard). red (cornelian), black, green (chrysoprase). black-and-white or gray—and-white (onyx): and yellow, red, brownish red or black for jasoers, which are semiopaque to opaque. All varieties are cut into cabochons, engraved, or made into seal stones or rounded, polished, and pierced for necklaces and other items of jewelry. Various forms of chalcedony were used extensively in the past for bases and handles of gold items (statuettes, goblets, cruets) and for stone inlay work. Agate and onyx, with their consecutive layers of different colors. make excellent material for cameos; the contrast between the different layers is used to heighten the relief.
Some variegated pieces are used for the carving of muiti colored figurines similar to those made from jadeite. The most highly prized variety nowadays is chrysoprase, which is a bright green color (commonly known as leek green).
Distinctive features The characteristic colors arranged in zones and the mid-level hardness and exceptional luster of the stones generally make identification of chalcedony quite easy. It is worth remembering, however, that it has a slightly lower density (2.61 g/cm“) and hardness (about 6.5) than other quartzes. As with all microcrystalline gems, it has only one refractive index (about 1.53), which is also slightly lower than that of most quartz.
Occurrence Large amounts of chalcedony come from Uruguay and the bordering regions of Brazil, but it is found in many other countries.
In Germany, ldar-Oberstein is famous for agate, although the term agate apparently comes from Akl-rates, the Greek name for a river in Sicily where these stones were found several centuries ec. Chryscprase comes mainly from Germany, the Soviet Union, the United States, Canada, and Brazil.
Value Its value was quite high in antiquity, when chalce dony was one of the main gems used. Nowadays, it is fairly low, except for chrysoprase, attractively colored specimens of which are quite valuable. Objects in chalcedony of considerable artistic merit tend to fetch high prices, Simulants and synthetics In the past chalcedony was imitated by glass, moulded pieces even being used to simulate carved stones. It is not produced synthetically, Because chalcedony is porous, it has long been the practice to impregnate it with artificial dyestuffs, making it look |ike onyx, even where the original color is almost uniform. This process is facilitated because the porosity of chalcedony often varies from one layer to another, so that one layer can absorb color well, whereas the adjoining one absorbs it little or not at all. Agate has been artificially colored for so long and the procedure is so widespread that i_t is regarded as normal, not fraudulent, practice. Chalcedony is also colored green to simulate chrysoprase. This practice is considered fraudulent as the value of chrysoprase depends almost exclusively on color. Equally fraudulent is intensification of the color of chrysoprase by the same means.