excellent agate article, includes staining agates, interesting!
AGATE, a term applied not to a distinct mineral species, but to an aggregate of various forms of silica, chiefly Chalcedony (q.v..) According to Theophrastus the agate (achates) was named from the river Achates, now the Drillo, in Sicily, where the stone was originally found. Most agates occur as nodules in eruptive rocks, or ancient lavas, where they represent cavities originally produced by the disengagement of vapor in the molten mass, and since filled, wholly or partially, by siliceous matter deposited in regular layers upon the walls. Such agates, when cut transversely, exhibit a succession of parallel lines, often of extreme tenuity, giving a banded appearance to the section, whence such stones are known as banded agate, riband agate and striped agate. Certain agates also occur, to a limited extent, in veins, of which a notable example is the beautiful brecciated agate of Schlottwitz, near Wesenstein in Saxony—a stone mostly composed of angular fragments of agate cemented with amethystine quartz.
Most commercial agate is artificially stained, so that stones naturally unattractive by their dull grey tints come to be valuable for ornamental purposes. The art of staining the stone is believed to be very ancient. Possibly referred to by Pliny (bk. xxxvii. cap. 75), it was certainly practiced at an early date by the Italian cameo-workers, and from Italy a knowledge of the art—long kept secret and practiced traditionally—passed in the early part of the 19th century to the agate-workers in Germany, by whom it has since been greatly developed. The coloring matter is absorbed by the porosity of the stone, but different stones and even different layers in the same stone exhibit great variation in absorptive power. The Brazilian agates lend themselves readily to coloration, while the German agates are much less receptive.
To produce a dark brown or black color, the stone is kept perhaps for two or three weeks in a saccharine solution, or in olive oil, at a moderate temperature. After removal from this medium, the agate is well washed and then digested for a short time in sulphuric acid, which entering the pores chars or carbonizes the absorbed sugar or oil. Certain layers of chalcedony are practically impermeable, and these consequently remain uncolored, so that an alternation of dark and white bands is obtained, thus giving rise to an onyx. If stained too dark, the color may be “drawn,” or lightened, by the action of nitric acid.
Agate is stained red, so as to form carnelian and sardonyx, by means of ferric oxide. This may be derived from any iron compound naturally present in the stone, especially from limonite by dehydration on baking. Some stones are “burnt” by mere exposure to the heat of the sun, whereby the brown color passes to red. Usually, however, an iron-salt, like ferrous sulphate, is artificially introduced in solution and then decomposed by heat, so as to form in the pores a rich red pigment.
A blue color, supposed to render the agate rather like lapis lazuli, is produced by using first an iron salt and then a solution of ferrocyanide or ferricyanide of potassium; a green color, like that of chrysoprase, is obtained by means of salts of nickel or of chromium; and a yellow tint is developed by the action of hydrochloric acid.
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