The distinction between this substance and agate rests upon very arbitrary grounds. Agate frequently presents a variety of colours, and a multiplicity of beautiful delineations. Calcedony is generally of one uniform colour, of a light brown, and sometimes nearly white. It occurs in irregular masses, forming grotesque cavities in the trap rocks, particularly in Iceland and the Faroe Isles, from the former of which there are specimens in the Museum of Edinburgh of a very large size. These stalactites appear always to have proceeded from the upper part of the cavity, which is sometimes filled up to the very summit with solid matter. From a close examination of these specimens, we are led to believe that the material must have been introduced into the cavity either in a state of the most attenuated fluidity, or even in a gaseous form. The structure can be traced down the sides of the cavity, regularly surrounding every portion of the stalactite, and passing across the horizontal plate which uniformly forms the base of these cavities. A slight intermixture of opalescent matter, which renders the calcedony more white and opaque, delineates this structure in the most perfect manner, and is a common occurrence in Faroe.

Calcedony is not confined to the trap rocks; it occurs in granite; and the most beautiful specimens known were found in one of the mines of Cornwall, distinguished by the name of Trevuscus. It was, however, in one solitary cell that these occurred. Although the mine has continued to yield calcedony of the same character, nothing similar to these magnificent specimens has been produced since. They can scarcely be compared to anything which they resemble more than the anatomized wing of a large bat, exhibiting the bones and arteries in the most perfect manner. One of these beautiful specimens, which was in the possession of the late Mr Greville, is now in the British Museum.

Calcedony is used for the construction of cups and plates, and other articles of taste, of which the most splendid specimens are imported into this country from India. The labour which has been bestowed in the manufacture of these articles, and the perfection with which it has been accomplished, is a matter of surprise to all who examine them. There are some of them as thin and as delicate as china. The finest stones are of course selected for this purpose. They are generally clear and almost transparent, still maintaining the warm brown colour which characterizes the stone. They often have the appearance of having been hammered, so shaded and undulated is the aspect of the mass; and to add to their beauty, the fine dendritic Mocha stones are often selected. We are in ignorance even of the locality where these beautiful objects are manufactured, whether in Japan or China; but to this country they generally come from India, where, we believe, they are found among the most precious jewels in the repositories of the nabobs and princes of the East.

Calcedony in Europe is confined to labours on a much smaller scale, such as knife-handles, and mortars for chemical purposes; also for snuff-boxes, buttons, and other minor objects. The principal manufactory is at Oberstein in the Palatinate.

Calcedony is semitransparent; its texture is fine and compact; the fracture is scaly, resembling that of wax; it is less hard than rock crystal, but gives fire with steel. No indications of regular form have ever been observed in this substance; for we need not except the pale blue variety from Treszyn in Transylvania, which is decidedly a pseudomorphic formation in the form of fluor spar. Calcedony frequently assumes the forms of other minerals, as well as of shells in many instances; but these may be considered as accidents unconnected with the history of the substance.

Connected with agate and calcedony, we may at once enumerate the different varieties which are still maintained by mineralogists.

1. Sardonyx (quartz, agate, sardoine of the French). This variety is characterized by a rich orange colour. 2. Carnelian (coraline of the French). The characteristic colour of this variety is a brilliant red. 3. Prase (the chrysoprase of the Germans). This variety occurs at Kosmitz in Silesi. It sometimes possesses a most brilliant green colour, and is nearly transparent. 4. Plasma, semitransparent, and of a dark grass-green colour. It was very much used by the ancients as a stone for engraving. 5. Onyx (the band-agate of the Germans). This stone, when of favourable colours, is used by artists for forming cameos. Mineralogists have split the agate into a variety of other denominations, which are not worth enumerating.