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Historic Methods of Artificially Coloring Agates

Last Updated: 13th Jan 2008

By Daniel Russell

Artificial Coloring of Chalcedony - Agate - Onyx

The artificial coloring of gem materials to make them more desirable to the consumer is as ancient as greed and avarice.

One of the earliest minerals to be “enhanced” by artifice was chalcedony, a variety of quartz which includes carnelian, sard, plasma, prase, bloodstone, onyx, sardonyx, chrysoprase, thundereggs, agate, flint, chert, jasper, petrified wood, and petrified dinosaur bone. Chalcedony is composed of microscopic fibers of quartz mixed with a small amount of moganite, a form of silicon dioxide different than quartz. The porous structure of many (but, as lapidaries discovered, not all) chalcedonies allow chemical solutions to infiltrate the stone, making it possible to create chemical reactions inside the stone itself. This creates a more durable color than merely coating the surface of the stone.

Pliny The Elder (born 23AD, died 79AD), in his Natural History, hints at ancient practices of gemstone enhancement, speaking of gemstones "which are boiled, it is said, in honey, for seven days and nights without intermission..." adding that once the stone is "thus cleansed and purified, is adorned by the ingenuity of artists with variegated veins and spots, and cut into such shapes as may be most to the taste of purchasers..."

While 19th century scholars of classical literature dismissed Pliny's comments as vague and fanciful, the simple technology of turning aesthetically unpleasing examples of chalcedony into more desirable colors was not lost by the gem industry.

In the 1823 edition of the Encyclopedia Britannica states that "agate may be stained artificially with [a] solution of silver in spirit of nitre [silver nitrate]…" followed by exposure to the sun.

A more detailed description of the process is offered by William Lewis a few years later:

Several of the hard stones which strike fire with steel, receive a dark stain, inclining to black, from solution of silver. M. du Fay [Charles François de Cisternay du Fay] relates, in the French Memoirs for 1728, that to chalcedony, this solution gave a reddish-brown colour; to oriental agate, a blacker stain; to an agate spotted with yellow, a purple…

Among the hard stones that have been tried, the agates seem to be those which are acted upon most readily; they are also those which have oftenest been attempted to be stained. The solution should be made in strong aqua fortis, or spirit of nitre (nitric acid), and be fully saturated with the metal. The stone, after the fluid is applied, should be exposed to the sun for two days or more; and if, when dry, it be removed into a moist place, and afterwards exposed again to the sun, the production of the colour will be the more speedy. After the stone has acquired the full colour which the first quantity of the solution can communicate, it may be moistened with more, and this repeated two or three times, by which the colour will be deepened, and made to penetrate further. M. du Fay found that an agate, about a sixth of an inch in thickness, by applying the solution on both sides, may be stained throughout its whole substance. The tincture, however, is rarely uniform on these or other stones; most of them having veins, which, though undiscernible in the natural stone, are in this process made apparent, being more easily or more difficultly penetrable than the rest of the mass, and sometimes forming not inelegant varieties in the stained stone. (Lewis, 1828)

To this he adds that, unlike the black of natural agates, the black of silver nitrate treated stones is damaged by exposure to high heat, and is completely dissolved away by soaking the stone in nitric acid overnight.

A more permanent and deeper black was afforded by soaking chalcedony in a copper nitrate solution. When the stones are placed in a crucible and heated almost to a red heat, the copper nitrate decomposed into black copper oxide. He says the color is durable but penetrates “only a very little way into the substance of the stones.”

One of the most detailed descriptions of the process to color chalcedony black by the “honey and acid” technique was published by W. Newton in the 1849 edition of the London Journal of Art and Science:
Among the various processes employed by the ancients for the coloring of these gems is one described by Pliny; but which, up to the present time, has been generally although erroneously treated as a fable: this process consisted in boiling the stone with honey, during at least seven or eight days; and it is a curious fact that this identical process is still employed in the agate manufactories of Oberstein and Idar, for the purpose of converting calcedonies and red and yellow cornelian into fine onyx. This singular process remained, during many years, a secret in the possession of an agate merchant of Idar, who had probably purchased it from the Italian and Roman artists, that were in the habit of frequenting that locality to buy stones suitable to their art. The coloring of these stones is founded upon the following property: The ribbons or zones in the different varieties of calcedony, which, in the kidney-formed masses of that substance, lie superimposed, differ in their texture and compactness ; but, owing to their similarity of color in the natural state, they can only be distinguished from each other with difficulty. The stone is however capable of absorbing fluids in the direction of the strata; this property the strata possess, however, in differing degrees; therefore, if a colored fluid be absorbed, and the quantity taken up by the pores of the stone is different for every strata or zone, it is clear that a number of tints will be produced, corresponding to the number of zones, each of which will indeed be rendered distinct and colored in proportion to the quantity of coloring fluid it may have absorbed; thus, a specimen of stone naturally but slightly colored may, by this treatment, be rendered equal to fine stratified calcedony or onyx, and may be equally well employed with them in the engraving of cameos, or for any other purpose where the variety of color can be rendered available.

The signs of value in these stones, when in their rough state, are recognized by the merchants by an empirical test, which rests upon the above-mentioned property of absorption of liquids. In the trial a small piece is broken off that part of the rough stone which is expected to be of marketable value when polished: this fragment is moistened by the tongue; the buyer then remarks carefully the rate at which the moisture dries away; or, rather, whether it be rapidly absorbed by the stone ; and also whether the absorption take place in alternate bands or zones, and in one zone more rapidly than in another. According to the greater or less rapidity of the absorption, the merchants judge of the aptness of the stone to receive color, and, above all, if it be likely to assume the appearance of onyx under the coloring process. The artificial coloring of these varieties of stone is practically carried on in the following manner: The stones about to be submitted to the coloring process are first washed with great care, and then equally carefully dried, but without exposure to an elevated temperature; when perfectly dry they are put into a mixture of honey and water — taking scrupulous care that the vessel employed be clean; above all, that it be free from every kind of greasy matter: a fire is lighted beneath the vessel, and the fluid heated rapidly; but, at the same time, ebullition must not be permitted; and the fluid, lost by evaporation, frequently replaced, in order that the stone be constantly kept covered — this is essential. The operation of the honey is continued for two or three weeks — the time necessary being known only by experiment. When it is considered to be complete, the stones are removed to another vessel, and strong sulphuric acid poured upon them until they are covered. A slab of slate is placed over this second vessel, which is then put upon a furnace, and the sulphuric acid heated to 350° or 400°. At the expiration of some eight or ten hours, the stones are generally found to have acquired the requisite color — that is to say, those that are at all capable of receiving this factitious coloring; for it will be found that some of the stones submitted to the operation will refuse the color entirely; indeed, in all, the effect varies very much. The larger and softer stones are finished in a few hours; but others require to be kept under the influence of the acid during the whole of a day. When finished, the stones are removed from the acid and thrown into water, where they are well washed, and then dried in a kind of oven; they are next polished, and, afterwards, put into oil, where they remain for a day or two, according to circumstances. The oil removes from the surface of the stone the appearance of slight flaws or fissures, and imparts to it a high degree of polish and brilliancy. The oil itself is removed by rubbing the stone gently with bran. The chemical action, which determines the access of color in this process, appears to be very simple: The honey penetrates into the porous layers of the stone, and is deorganized and carbonized in the pores by the sulphuric acid. The color of the bands, which absorb the honey, is thus more or less increased by the deposition of the carbon. The colors, which naturally were barely indicated by different degrees of transparency in the zones, become, by this treatment, grey, brown, or even almost black ; whilst the white parts are rendered brighter and more distinct by becoming, under the influence of the high temperature, more opaque. This also the case with the bands of red; so that, not only is color given where none previously existed, but even those parts that were originally colored acquire a brightness of tint and distinctness of marking much greater than that which they naturally possessed.”

A year later, Swiss mineralogist Johann Jacob Nöggerath, director of the Bonn Museum of Natural History, committed his observations on the process to paper in an article in the Edinbrugh Philosophical Journal. [In addition to amplifying on Newton's comments, Nöggerath makes an important observation on how Germany lapidaries judge the suitability of specimens for "colorization": "They break off a thin portion of a seemingly useful mass, and after moistening it with the tongue, observe whether the absorption of the moisture by the alternate bands takes place at regular intervals" This may, in fact, be the first formal description of an "agate licker" in the gemological literature!]

Nöggerath expands on the methods used to "enhance" chalcedony. In addition to the ancient practice of blackening chalcedony using honey carbonized by sulfuric acid, he adds the following practices:
• "burning" chalcedony in order to enhance color (commonly used to create a carnelian-like appearance);
• creating citrine-colored chalcedony by treating iron-oxide containing stones with hydrochloric acid, resulting in a valence change from Fe3 to Fe2;
• the creation of a blue color that simulates "all the different shades of the turquoise" by a process he was unable to determine - it is uncertain whether the technique is related to the "ferrocyanide method" described by both English and Dreher, below, or the result of treatment with copper sulfate followed by ammonia.
In the last century many experiments were made to colour agate, calcedony, carnelian, &c., by solutions of metals, &c, applied to the surface, and sometimes made to penetrate slightly into them. The processes have been frequently described, but it remained unknown how to render the various quartzes included among the gems of the ancients penetrable to colouring fluids.

Within the last twenty or twenty-five years the processes of the agate-cutters of Oberstein and Idar have reached such perfection that they are able not only to bring out and heighten the natural colours of calcedony, onyx, carnelian,&c., which are sometimes faint, but also to render them entirely penetrable to colouring fluids by which the beauty and variety of the stones is much increased.

This colouring process was at first a secret, known only to a few agate-dealers in Idar. It was eagerly sought after by Roman stone-cutters (as the lapidaries of Oberstein say), who bought up all the onyxes. The secret seems at length to have been discovered by some of the foreigners, or been bought up.

This art arises out of the property which the fine layers of calcedony, although exhibiting only faint differences of colour, possess of becoming variously coloured by the application of colouring fluids. By this process very mean-looking slightly-coloured stones can be turned into very fine onyxes, &c, which by their various bands of colour afford materials for cameos; and at least the beauty and designs of the agates intended for other purposes are much increased.

There is a method by which the agate-dealers of Oberstein and Idar determine the fitness of the crude minerals for the colouring process; at least to value them before purchasing them from the diggers. They break off a thin portion of a seemingly useful mass, and after moistening it with the tongue, observe whether the absorption of the moisture by the alternate bands takes place at regular intervals; if so, it is then deemed fit to be coloured as an onyx. This proof is not always decisive of the value of the minerals, yet it affords a fair criterion to the dealers to go by before they buy valuable pieces from the diggers. Large balls of calcedony, in which many fine bands occur, especially if the rest be of a red colour, are much prized. Weisselberg, near Oberkirchen, in the district of Wendel, produces fine specimens, but in small quantity. Ramstedt relates that one was found in the year 1844, and weighed 100 lb. It was sold in its crude state for 700 Rh. guilders.

The purchases between the diggers and dealers are made by mutual understanding, and generally without any previous trials being made or desired, the price being agreed on by the weight. That the different varieties of quartz which form agate balls and amygdaloidal-shaped pieces, vary in their porosity has been proved by an interesting experiment of Von Kobell, who applied [hydro]fluoric acid to polished agate where the different streaks were not regularly developed and only slightly visible...

In many transparent calcedonies, the little cavities which the stone contains may be recognised by the naked eye. They are seen to be small bubbles often round, often long, frequently running into each other, and forming tuberculous cavities. In others, however, they cannot be observed by the naked eye, but are easily seen by the aid of a microscope, under which they appear to be filled with small cavities, especially the Brazilian Carnelian, which is particularly well suited for colouring...

The colouring of onyx and calcedonyx (if we are to understand that the white and black, or dark-brown, stones are to be called onyx, and the white-and-grey streaked varieties are to be called calcedonyx) is performed at Oberstein and Idar in the following way. The best stones are first well washed and dried without any raising of the temperature ; after this they are placed in honey diluted with water (one half pound of honey to a chopin of water). [A chopin is an obsolete Scottish fluid measurement equivalent to about half a pint – DER] The pot in which they are then to be placed must be clean and free from grease. It must then be put into hot ashes, or into a hot oven, and the stones covered with the fluid, which must not be allowed to boil. The minerals must, indeed, always be covered with the fluid, which must be added from time to time. The minerals are to be treated in this way for a fortnight or three weeks. They are then taken out of the honey, washed, and placed in another vessel along with as much oil of vitriol [sulfuric acid – DER] as will cover them. The vessel is then to be covered with a lid, and placed in ashes in which hot coals are placed. The porous or soft stones are coloured in an hour, others in a day, and some take on no colour at all. The stones are taken out, washed, and placed in an oven. After which they are ground, and kept a day in oil, by which some fine cracks are made to disappear, and a better polish obtained; the oil is then rubbed off with bran. By this process light grey streaks are brought out on some; and others, according as their porosity was greater or less, indicate grey, brown, or black streaks. The white impenetrable masses become whiter through the loss of their transparency, and many red streaks become heightened.

The so-called Carneóle [carnelian,]from Brazil, which is wrought in Oberstein and Idar in great quantity, costs, on an average, about 50 guilders the 100 lb. Those selected with straight streaks, as being suitable for cameos, often cost as high as 2500 guilders per cwt., receive sometimes the same treatment as the native stones, and sometimes the process employed in colouring carnelian and sardonyx, as I will shortly relate.

They are originally either one-coloured, muddy yellow, grey, or contain a variety of shades of such colours, and can scarcely be called carnelian in their natural state, which name is only given to such as are of a red colour. These carnelians, when found with streaks, after they have received the above-mentioned treatment, form the finest onyx. The chemical changes induced by the above-related processes require no detailed explanation. By the placing of the stones in hot honey, the latter penetrates into the fine pores of the stone; the vitriolic acid then causes carbonization of the animal substance — and the more the honey in the stone is carbonized the darker its colour becomes; and while the slightly porous portions become only grey or brown, the more porous ones become black. The white and red bands appear not to be penetrable to the honey, and it is to the treatment alone that we can attribute the increased intensity of their colours. Brazilian carnelian contains the oxyhydrate of iron, and is generally penetrable in its bands; the red tints are destroyed by the carbon, and appear of the colour of a mixture of grey and black, or most commonly of a dark brown. These Brazilian carnelians afford the finest onyxes.

Calcedony can be coloured a very fine citron-yellow, either generally diffused or streaked (if this condition is already indicated in the stone). The process is as follows: they are first dried two days in an oven, care being taken not to let the oven become too warm; the stones are then to be placed in a clean vessel, and covered with spirit of salt [ hydrochloric acid – DER]. A cover must be firmly cemented on the vessel with clay; they must then remain from fourteen days to three weeks in the oven, and then the yellow colouring process is complete.

It deserves further inquiry, whether this yellow colour is occasioned by the formation of a salt — by the mixture of the hydrochloric acid with some previously existing matter in the stone itself, or whether the colouring principle is entirely contained in the acid. Ï know no natural calcedony having a colour similar to that produced in this way. There occurs, however, in opals such a citron-yellow colour, but it is rather more of the appearance of wax. In the coloured stones, however, this shade shews itself here and there, and seems to be inherent in them, as the colouring matter always remains the same.

Of late years a very fine blue colour has been produced in calcedony, shewing all the different shades of the turquoise. The process for this is yet a secret, known only to a few of the cutters.

Many minerals are also burned — such as agate, calcedony, and Brazilian carnelian. This is done partly to increase the beauty of their natural colours, and partly, as it is said, to give the natural colours more durability. Many calcedonies become, through this process, almost white, the red colours more intense, and the pale yellow a very fine red. This is also the case with the Brazilian carnelian. By which process the streaked stones of this kind become transformed into fine sardonyxes, and those with one colour take on the true colour of the carnelian. The process is as follows: the stones are rendered perfectly dry, by being placed for a fortnight or three weeks in a hot oven; they are then placed in a shallow dish and moistened with vitriolic acid, but not covered. The polishers usually dip the stones in the acid, and then place them beside each other in a Teasel, which is then covered and placed in a hot fire until they are red hot. The fire is slowly extinguished, and they are taken out when cool; by this roasting the oxyhydrate of iron which the stones contained is freed from its moisture, and the colour of the oxide assumes a more lively hue, and is seen in the translucent mass in the proper colour of the cornelian...

There are many other dexterous manipulations necessary, which are known only to the polishers themselves, but I have collected the above processes from many sources; and my esteemed friend, Herr Tischbein of Herstein, in the Palatinate of Birkenfeld, has given me many particulars which assisted my studies on the agate very much. (I acknowledge them here with much thankfulness.) When once, however, the properties which these minerals (to which I have given the collective name of agate) possess of being quite penetrable to colouring fluids, in consequence of their porosity, are better known, it is probable that other colours may be given them; and, also, that many antique stones, presenting unusual colours, may have been coloured so artificially. This seems to me very likely, as many of the antique cameos and intaglios which I have seen in collections seem to be so. (Nöggerath, 1850)

Dekalb, writing in 1896, suggests that another method of coloring chalcedony is in use: creating a reddish brown color by soaking the stone in ferric chloride, then precipitating iron oxide in the microscopic interstices of the chalcedony by treating it with ammonium hydroxide.
The true, or precious onyx, is distinguished arbitrarily from the agates by the perfect parallelism of the color-bands, these bands consisting usually of alternations of white and black, white and brown, and white and red. It may be mentioned, in passing, that such perfect banding is so exceedingly rare that very few, if any, of the onyxes or cameos sold in our jewelry shops are from naturally-colored stones, the artificial coloring of agates being a regular industry in Germany. The method is said to consist in saturating the more porous layers of the banded white or bluish slate-colored agate with honey, and then carbonizing this with sulphuric acid, to produce the black-and-white variety. The red-and-white is produced by soaking in ferric chloride and precipitating ferric oxide with ammonia. Such a red coloration should consequently be obtained in any porous stone by similar treatment; but my personal experiments in this direction have been unsuccessful, or, at best, have resulted in a dirty ferric-oxide stain. (Dekalb, 1896)

Mineral dealer George Letchworth English helps fill out the color-wheel of artificial chalcedony colors by adding green - the result of treatment with nickel or chromium compounds - and an explanation of the manner in which blue chalcedony was achieved. This process involved treating the stones with potassium ferrocyanide following by iron sulfate to form "Prussian blue" (ferric ferricyanide).
Nearly all the agates now offered for sale are, however, artificially colored. The success of the process is due to the varying degrees of porosity of the different 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 certain of the layers, making them dark brown or black. The red, or carnelian agates are produced by a process of “burning”. A grayish stone is heated in an oven for several weeks, at first gently, then it is moistened with sulphuric 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 secured 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. (English, 1904)

F. W. Rudler (then formerly of the Museum of Economic Geology in London) writing in the entry on Agate in the Dictionary of Applied Chemistry, also noted that the color of carnelian could be imitated if the chalcedony were "placed in a solution of ferric nitrate, prepared by throwing old nails into dilute aqua fortis... the stone is afterwards exposed to a red heat, whereby the absorbed salt is decomposed and ferric oxide formed." (Rudler, 1912)

In 1913, O. Dreher published a detailed account of the processes used in Idar, Germany in the 19th Century to color chalcedony (Dreher, 1913). His father and other family members had apparently been involved in the lapidary arts in that city.

Dreher claimed that the process of coloring chalcedony black was first introduced to Idar in 1819. He said that the practice was commonly called “the honey bath” by Idar’s lapidaries. It followed the time-honored practice of carbonizing a sugar that had permeated the interstices of the chalcedony… but rather than using honey, the Idar lapidaries switched to less expensive common household sugar (about 375 grams dissolved in a liter of water). He also noted that some of the stones, after the sulfuric acid bath, a percentage of the stones tend to “sweat” small amounts of sulfuric acid… which is easily prevented by soaking in warm water for a few hours.

“Burning” Agate
The process of enhancing the color derived from minute traces of iron naturally found in chalcedony by “burning” (heating the chalcedony in a furnace) was, according to Dreher, begun in Idar about 1813. He claimed that lapidaries had observed that some agates recovered from fields showed a distinct carnelian color in the portion that was protruding above ground, while the portion below the surface remained colorless. Heating sped up the process of converting colorless traces of iron into visible iron oxides.

Happily for the people of Idar, the process of “burning agate” in that city did not involve setting flame to enormous masses of sheep dung, as was done in the Indian lapidary center of Khambat, India (see The Carnelian Mines of Rajpipla for more details on this fragrant process) Instead, the stones were first baked in an oven at a temperature just high enough to drive out any moisture (which could take from 2 to 10 days), then packed in a crucible surrounded by magnesite or asbestos (in addition to separating the stones, packing a refractory material also slows down the rate of cooling and heating of the stones, reducing the chance that the stones will be shattered by thermal shock). The crucible was placed in a furnace and the temperature slowly raised to red heat, then the crucible was allowed to slowly cool. Stones that didn’t achieve the desired red color were soaked in iron nitrate and “recycled” through the burning process again.

Dreher states that two different blue colors were being produced by Idar lapidaries. The first was the classical “Prussian blue” color created by soaking the chalcedony in “yellow prussiate of potassium” (potassium ferricyanide) and then treating it with iron sulfate to form iron ferricyanide.

The second method Deher describes is similar, except that potassium ferrocyanide — the “red prussiate of potassium”instead of ferricyanide — is used to create a “Turnbull blue.” (It is claimed by some artists that there is a slight difference in hue between Prussian Blue and Turnbull Blue… clearly, I must be color blind!)

It is especially interesting that Deher claims that the “Prussian blue” process was introduced to Idar in 1845. Previously, in 1842, German-born British astronomer-chemist Sir William Herschel (discoverer of Uranus) announced a chemical process where materials soaked in a solution of potassium ferrocyanide and other iron salts were made photosensitive… leading very quickly to the widespread use of “blueprinting” to copy drawings. Suddenly, potassium ferrocyanide was the “process celeb” throughout the scientific community, which is suggestive that this discovery spurred Idar’s more chemistry-minded lapidaries to experiment with potassium ferrocyanide.

Dreher states that, regardless of whether potassium salt — ferrocyanide or ferrocyanide — is used, the Idar lapidaries used a solution composed of 250 grams of the salt to a liter of water. After soaking for a week or two in a hot, but not boiling, bath of the potassium ferrocyanide, the stone is placed into a second bath of iron sulfate for four to eight days, depending on the depth of color desired. He adds that a darker blue color will be achieved by adding a few drops of nitric or sulfuric acid to the iron sulfate bath. [Important notice: acid must NEVER be combined with the potassium ferricyanide or ferrocyanide solution, as there is a genuine risk of generating cyanide gas.]

According to Dreher, creating brown color in chalcedony is accomplished by the simple artifice of soaking the stone in a sugar or honey solution, then heating it just enough to caramelize the sugar. (It would be interesting to determine how permanent this process is!)

Green colors were created using chromium salts or nickel salts. Dreher says that green was introduced as a colorant for chalcedony in Idar in 1853, although whether this date represents the beginning of chromium or nickel based colors, or both, is uncertain.

A bluish-green was created in chalcedony by chromium salts. The stone was initially soaked in either chromic acid or potassium dichromate (Dreher points out that the lapidaries seemed to prefer chromic acid even though potassium dichromate was cheaper.) Thinner stones would be soaked for eight to fourteen days, although Dreher said in practice stones more than 10mm in thickness were soaked for up to six to eight weeks. The stones were then dried and placed in a warmed container containing solid ammonium carbonate for two weeks. This created an ammonia gas atmosphere around the stones, which helped “develop” the color further (an ammonium hydroxide solution was not used as it might wash out the extremely soluble chromium salts). The stones were then “burned” to fix the color.

The nickel process involved saturating the stone nickel nitrate, then “burning” the stone to create nickel oxide in the interstices. The final product supposedly imitated the bright apple-green of chrysoprase, a naturally-occurring chalcedony colored by trace amounts of nickel.

In addition to the processing “burning” agate to create a red color in chalcedony that contained traces of iron, the early Idar lapidaries developed a process to color chalcedony which contained insufficient natural iron. The stone was soaked for one to four weeks in a warm solution of iron nitrate (which, Dreher says, the old-time lapidaries said should be as thin as Munich beer). The lapidaries developed a rule of thumb about how long the stones should be soaked, based on thickness: stones 3mm thick were soaked for a week, while stones 6mm thick were left in the iron nitrate bath for three weeks. Stones 10mm thick required four weeks. Dreher complained that the iron nitrate solution seldom penetrated stones greater than 10mm thick to a sufficient depth to color the whole of the body of the stone. The stone was then “burned” to transform the iron nitrate into iron oxides.

Dreher makes no mention of methods to create yellow color in chalcedony. However, other writers pointed out that yellow chalcedony was the single least favorite color of the buying public, so it is likely that Idar’s lapidaries expended little effort in that direction.

Organic Dyestuffs
During the second half of the 19th Century, the synthesis of coal-tar derived aniline dyes (later found to be highly toxic and possibly carcinogenic) created an explosion of color throughout the western world. Suddenly brilliant, vibrant colors were available for textiles, wallpapers, inks and paint that had never been possible to achieve before. Dreher said that Idar lapidaries had employed aniline dyes but noted that they were not as permanent as the old, established methods of coloring chalcedony. The process was in use at least as early as 1884, when it receives a passing mention in the New York Times.

The use of aniline dyes to color agate appears to continue, as several recent papers on methods of decolorizing wastewater from the agate factories of Rio de Sul, Brazil have been published as recently as 2006 (as an example, see Pizzalato, et alia, 2002; and Barros, 2006)


• Barros, A. L., Pizzolato, T. M, Carissimib, E and Schneider I.A (2006) Decolorizing dye wastewater from the agate industry with Fenton oxidation process. Minerals Engineering Volume 19, Issue 1, January 2006, Pages 87-90

• DeKalb, Courtenay “Onyx-Marbles. “
Transactions Of The American Institute Of Mining Engineers. Vol 25, pp 258 (1896)

• Dreher, O. 1913 Farben des Achates: Idar , p. 20. Translated by Farrington, O. C., 1926, reprinted in The Mineralogist, v. 7, no. 8, p. 331-332, 345-346.

• English, George Letchworth, “Agate” in: Beach, Frank Converse, ed. The Encyclopedia Americana New York 1904 Chicago

• Lewis, William “On the Different Black Colors”. Gill’s Technological Repository, Vol. 2 pp 289 – 291. London 1828

• Newton, W. “On The Artificial Coloring Of Cornelians, Calcedonies, &c”
The London Journal of Art, Science and Manufacture Vol 35 pp 347-348
London, 1849

• New York Times “Concerning Agates”, 28 July 1884

• Nöggerath, J J “On the Porosity and Colouring of Agates, Calcedonies, &c.”
In: The Edinburg Philosophical Journal Vol 48 (1850) pp166-172

• Pizzalato, T.M.; Carissimi E.; Machado E.L.; Schneider I.A.
Colour removal with NaClO of dye wastewater from an agate-processing plant in Rio Grande do Sul, Brazil International Journal of Mineral Processing, Volume 65, Number 3, July 2002 , pp. 203-211(9)

• Rudler, F W "Agate" in: Thorpe, Thomas Edward, ed. Dictionary of Applied Chemistry
London 1912

Completed: 11 January 2008
Major Revision: 13 January 2008

Article has been viewed at least 24671 times.


I just love Daniel Russell's article here on
I saw this website that shows exactly how to dye agates

It has the recipes, temperatures and treatments listed, and links to the chemicals you need to do the dyeing yourself.

Justin Zzyzx
27th Dec 2012 10:26am

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