Chalcedony (from the city of Chalcedon), one of the numerous varieties of the quartz family, which are distinguished from each other, not by difference of chemical composition, but by their external form, markings, and colors. The peculiarities of chalcedony consist principally in its mammillary, botryoidal, and stalactitic shapes, and its waxy or horny lustre and texture. It is found lining cavities in trap, and also in other rocks, being arranged in concentric layers, precisely as if its particles had been introduced in a gaseous or fluid form. The intermixture of coal with the purely sili-cious layers suggests that water was present during the production of the mineral incrustation. By the variety of its colors and the high lustre it acquires by polishing, chalcedony is much esteemed as an ornamental stone,though its great hardness renders it very difficult to work. In several of the countries of Asia it is common to find articles of this stone, as cups, plates, etc, of the most exquisite workmanship, as delicate as the finest chinaware. Specimens of the finest texture and most delicate shades are selected for these, especially such as are more or less white, passing into transparent and brown.

At Oberstein, in Germany, chalcedony is worked like agate. (See Agate.) Some of the finest known specimens of chalcedony were found at the Tresavean copper mine in Cornwall. They occurred in a single pocket or cavity in the mine, and none others were found like them. One of them, described as resembling the anatomized wing of a large bat, displaying its bones and arteries, is preserved in the British museum. The mineral is frequently met with in the United States, and is particularly abundant where metallic veins are worked, but no specimens of extraordinary beauty are found. It also occurs in Nova Scotia and Iceland.