AGATE, a variety of chalcedony, or cryp to-crystalline quartz, distinguished by its banded or clouded appearance or by the pres ence of visible impurities.
The method of formation of banded agates has been carefully studied by numerous in vestigators. M. F. Heddle (consult Natters, Vol. 29, p. 419) assumes the existence of a cavity in a trap rock which is lined during its solidification with a thin layer of green cela donite or delessite. The rock subsequently rots, and its feldspar is decomposed by water con taining carbonic acid, which thus becomes highly charged with silica. This is transfused into the cavity, coagulates and is deposited on its walls. The banded structure is due to the intermittent deposition of successive layers of silica from the highly siliceous solution. It was long supposed that after the earlier layers were deposited fresh supplies of the solution passed inward through a hole called the *inlet of infil tration.* This canal is often distinctly seen in agates, but the weight of evidence now seems to favor the hypothesis that the external solu tion supplies the silica by osmosis (q.v.). The internal solution becomes less dense as it is re lieved of silica by its deposition on the walls of the cavity, and it is then replaced by the denser external solution which passes through the layers of agate already formed. If the condi tions remain unchanged the process continues until the entire cavity is filled, but agates are not infrequently found with a cavity in the centre, or with crystallized quartz (often ame thystine) lining the interior. Such distinctly crystallized portions may be succeeded by fur ther deposits of the cryptocrystalline chalce dony. The successive layers often differ much in density, hardness, color and transparency. Sir D. Brewster has shown that some of the layers are so exceedingly thin that it would take 55,760 to measure an inch. (Philosophical
Magazine (3) XXII, p. 213). These layers are deposited all around the cavities, following all of their irregularities. The beauty of agates is thus greatly enhanced by extreme delicacy of their banding. Sometimes these bands concen trically encircle a dark spot, forming a °bull's eye agate,* or again they parallel a cavity with sharp angles, as in the (4 fortificat io n agate.* Other agates show horizontal layers, suggesting the query as to whether the silica had settled upon the floor of the cavity from a soltition at rest. Such agates are known as onyx (q.v.), or if the colors are red and white, as sardonyx (q.v.). In clouded agates there is no distinct banded structure, but the colors shade grad ually from one into another.
The natural colors of agates vary greatly, being usually gray or either red or brown. The colors are due to the presence of organic im purities or to the oxides of iron, manganese or titanium. Nearly all the agates now offered for sale are, however, artificially colored. The success of the process is due to the varying de grees of porosity of the crtfferent layers of agate, some of which readily absorb the fluid in which the stones are immersed, while others are impervious to it. The black and white agates are prepared by soaking the stones for several days in a warm syrup of honey and water, then immersing them in sulphuric acid, which carbonizes the honey absorbed by cer tain of the layers, making them dark brown or black. The red or carnelian agates are pro duced by a process of *burning.° A grayish stone is heated in an oven for several weeks, at first gently, then it is moistened with sul phuric acid and the temperature is gradually raised to redness. Blue or *sapphire* agates are produced by steeping the stones first in a solution of a ferric salt and then in potassium ferrocyanide, thus depositing Prussian blue in the more porous layers. A green agate is se cured by the aid of chromic acid or a nickel salt, while hydrochloric acid yields a yellow agate. The red and the black are much the most popular.