Gemstone Coloration and Dyeing – Pinks and Reds

There are only a few inorganic processes, in my experience at least, that will impart pinks or reds to gemstone. These involve primarily four metals; cobalt, iron, mercury and silver. We already know cobalt can induce blue, purple, amethyst, etc. and incidentally pink in gemstone.

Color Inducing Compounds

There are also two additional cobalt processes that impart pink or lavender, primarily or exclusively. Several iron compounds are red or reddish, common rust being a familiar example and at least one of these is useful in coloring gemstone. The same is true of mercury. And there are two red-silver compounds that are adaptable to coloring gemstone.

12. Cobalt Chloride

Materials needed:

  • Cobalt chloride (cobaltous chloride), CoCl26H20


This is a one solution, one-step process. Prepare a saturated solution of the cobalt chloride. This will take about one pound of the crystals to a quart of water. Immerse the clean, dried slabs individually in this solution for a minimum of two weeks, preferably three.

Pour off the solution and store for re-use, indefinitely.

Rinse and dry the slabs (in the open air, not in the oven). This process produces some beautiful pink and lavender hues on some gemstone varieties. It works well with Mexican dendritic, crazy lace, polka dot (white), stinking water plume, snakeskin and troyite agates.

On the other gemstone varieties, the coloration is too pale or maybe lacking entirely. The color imparted to snakeskin agate is especially nice if you like pink because a rich translucent pink is produced, suggestive of a good grade of rose quartz. Polka dot is also richly pink but not translucent. The opalite portions of troyite sometimes color almost as deeply pink as rhodochrosite.Other cobalt salts can be used e.g. acetate, nitrate and sulfate, but I think they are inferior to the chloride and are more expensive.

Cobalt acetate and sulfate both produce paler colorations than the chloride. Cobalt nitrate will color gemstone as well as the chloride will but does not seem to be as light fast. In direct summer sun over a period of a few weeks, agate slabs that have been colored with cobalt nitrate take on a grayish overtone. I suspect that this is due to the heat of the summer sun rather than the light itself. If agate slabs such as polka dot are overheated in the oven in the process of drying them, they will turn almost black.

Theoretically at least, this is due to the partial breakdown of the cobalt nitrate from the heat of the sun to form cobalt oxide, which is dark gray to black.This cobalt chloride process is precisely the same as the primary soak (procedure A) of Processes No. 7, 8, 9, 10 and 13 below. Thus, a single batch of slabs soaked in cobalt chloride solution can be used for six separate processes (7 -10, 12 and 13 below).

13. Cobalt Chloride – Sodium Phosphate

Materials needed:

  • Cobalt chloride (cobaltous chloride), CoCl26H20
  • Sodium phosphate (sodium orthophosphate), Na2HPO4 · H2O


The procedure for this process is precisely the same as for Process No. 10 except that the ammonia is omitted. Simply use the sodium phosphate without any ammonia added.

The coloration induced by this process is much like that of Process No. 12 above, cobalt chloride alone except that it is somewhat darker and tends to be more lavender than pink. The same gemstone varieties color well with this process as with Process No. 12 above.

14. Iron Nitrate

Materials needed:

  • Iron nitrate (ferric nitrate), Fe(N03)3


Prepare approximately a saturated solution of the iron nitrate. It is very soluble, so add warm water sparingly to the crystals until they are almost all dissolved. One pint of water will dissolve almost two pounds of iron nitrate. It will be noticed that the freshly prepared solution of iron nitrate is quite cold, even though warm water is used. This is because dissolving iron nitrate in water results in an endothermic reaction whereby heat is lost.Immerse the clean, dried slabs in this solution for at least two weeks.

Slabs of coconut agate (and any others with calcareous or limy areas) should be given the acid treatment first. Otherwise the iron nitrate will react with the calcareous portions and the iron nitrate will eventually be decomposed.

After the soaking in the iron nitrate solution, drain of f the solution and store for re-use. Rinse the slabs and dry in the oven at lowest heat for at least twenty-four hours. The slabs are now ready for packing in the “sand bath”, preparatory to heat treatment. Follow carefully the directions for sand bath packing the slabs and subsequent heat treatment.

The heat treatment in this process breaks down the iron nitrate in the pores of agate or other susceptible gemstone to form one of the iron oxides, probably closely akin to, if not identical with, ordinary iron rust, and this is the source of the red hues produced. The color that is induced is not a scarlet or carmine, but tends to be more a brownish or brick red. Nevertheless, many gemstone varieties are much improved by the process.

Notable among these, in my opinion, are coconut agate, oolite, Mexican fortification agate, crazy lace, moon agate, snakeskin agate, stinking water plume andPriday Ranch thundereggs. In fact, all the “fortification” types seem to respond well to this process in that the coloration accentuates the design or pattern in the fortifications.Some of the polka dot agate, as it occurs in the polka dot bed, is just plain brown.

By the ferric nitrate – heat process, the brown polka dot agate is improved to a brownish-red. The white polka dot agate is not so improved; it takes on a dull or dirty red. The results with snakeskin agate tend to be variable for no obvious (to me) reason. Sometimes a very attractive carnelian color is induced, sometimes an opaque brick red and sometime little or no color at all. Mexican dendritic agate responds disappointingly to this process. At best, only an unattractive, splotchy, dull brown is produced.

In Process No. 14 and also in No. 23 where strong heating is involved, another physical change besides color takes place. The agate seems to be “opalized”. That is, it becomes a bit (not seriously) brittle. The broken surface is seen to be slightly glossy like opalite or opalized wood rather than dull or non- glossy like agate. However, this physical change imposes no hardship on the lapidary. Cabochons can be processed from such slabs with the same techniques as commonly used with unheated agate and with equally pleasing results.

15. Ammonium Dichromate – Silver Nitrate

Materials needed:

  • Ammonium dichromate, (NH4)2Cr2O7
  • Silver nitrate, AgNO3
  • Distilled water


  1. A. Prepare a dilute solution of the ammonium dichromate at the rate of four ounces of the crystals dissolved in one quart of water. Soak clean, dried slabs in this solution for at least two weeks. Pour the solution of f and store for later re-use. It can be re-used indefinitely. Rinse. Give final rinse in distilled water. Dry the slabs in the oven at the lowest possible temperature (approximately 140°F).
  2. Prepare a dilute solution of the silver nitrate at the rate of one fourth ounce of the crystals dissolved in twenty-four ounces (a pint and a half or a “fifth”) of distilled water. Be sure the container is clean, and give final rinse with distilled water before making the solution in it. Prepare preforms of desired shapes and sizes from the slabs from procedure A above. Shape and grind at least through the 220 grit stage. Wash the cabochons in detergent solution (common “dishwater”). Rinse thoroughly and give final rinse with distilled water. Transfer the cabochons from the distilled water to the silver nitrate solution without touching them. Use stainless steel tablespoon (clean!) or rubber gloves. Soak the cabochons in the silver nitrate solution for twelve to fourteen days. Drain of f the solution, rinse and then soak the cabochons in distilled water for a week or ten days. Dry the cabochons, dop them, and they are ready to sand and polish.Success with this process is complicated by the fact that in the presence of any organic matter in the solutions or in the gemstone material itself, silver nitrate tends to react with even minute amounts of sulfur compounds to form silver sulfide which will manifest itself as minute blackish particles in the slabs or cabochons in the silver nitrate soak, thus obscuring the coloration that the process produces. This is why we try to guard against the instability of silver nitrate in the presence of even traces of organic matter by using distilled water for making the solutions and for the final rinsing of the cabochons. The purpose of soaking the colored cabochons from procedure B in distilled water is to wash out any residual silver nitrate that was not converted to the brick red silver dichromate.This is one of two processes (No. 16 is the other) where it is advisable to at least partially finish the desired cabochons before completing the color process. The reason for this is that the reaction between the silver nitrate and the ammonium dichromate produces a brick red, insoluble precipitate right in the pores of the gemstone material, and penetration of the reaction is very slow requiring as much as ten to twelve weeks, or even longer in some materials. Therefore, it is more practical to mark out the cabochons on the slabs after they have soaked in the ammonium dichromate and before the silver nitrate treatment. The yellow color in the slabs after they have soaked in the ammonium dichromate is sufficient guide for selecting good cabochon areas, for what is yellow will be more or less a rich orange red to brick red after the silver nitrate soak. Since deep penetration of the partially finished cabochons is not essential for effective coloration, it is necessary to leave them only long enough to get the desired color intensity. This usually requires only two weeks or less.Other chromates and dichromates can be used. I have used both the chromates and the dichromates of sodium, potassium and ammonium for this process. Theoretically, it should matter little or none which is used, but I have had best success with sodium dichromate and ammonium dichromate with a slight superiority evident in the latter.

16. Mercuric Chloride

Materials needed:

  • Potassium iodide, KI
  • Mercuric chloride (bichloride of mercury, corrosive sublimate), HgCl2 Poisonous


  1. Prepare approximately a saturated solution of the potassium iodide. About two pounds of the crystals dissolved in a quart of water is required. Immerse clean, dried slabs in this solution and allow them to soak for at least two weeks. Drain of f the solution and store for re-use. Rinse and dry the slabs.
  2. Prepare approximately a saturated solution of the mercuric chloride. (Caution: This stuff, if taken internally, even in minute amounts, can be deadly poisonous.) This will require about eight ounces of the chemical to a quart of warm water. At this point, you will need to decide whether to color slabs or cabochons, and I would recommend the latter. As is the preceding process, color penetration is very slow requiring several weeks. Such a long wait can be circumvented by roughing out the desired cabochons from the slabs coming from procedure A. They will have a pale, greenish-yellow color but deep enough to be able to anticipate what the final result will be. Select cabochon areas to take best advantage of existing pattern, design and color intensity and scribe the cabochons accordingly. Carry the cabochon process through the fine grind (220 grit).

Carefully immerse these cabochons (or slabs if you go that route) from procedure A in the mercuric chloride solution one at a time so as to avoid splashing. If you get any on your fingers, wash it of f immediately lest you forget to later. Allow the cabochons to soak ten days or more or until you are satisfied with the color. If you are coloring slabs, allow at least four weeks, preferably more for complete color penetration. They will begin to take on an orange-red color very soon and will give the illusion that the coloration process is rapidly nearing completion. Do not be misled, however, because penetration is very slow and may not be complete even after four weeks. You can test this by snipping of f a corner of a slab and inspecting it closely for extent of penetration. If not complete, continue the soaking another week or two.

Always bear in mind that the mercuric chloride solution is extremely poisonous if taken internally, so if you get any of it on your hands wash it of f immediately. When the soaking is complete, pour of f the solution and store for re-use, well labelled ” Poison ” and away from children. Soak the slabs or cabochons in water (running water is ideal) for several days to wash of f any residual mercuric chloride. Dry the slabs and they are ready for use.

This process produces bright orange-red mercuric iodide in the slabs (or cabochons). It is insoluble (in water) and the slabs or cabochons impregnated with it are therefore, safe to handle.

Probably no other process described in this book yields coloration so subject to individual appreciation. Some people consider it “gorgeous”; others consider it “gaudy”. Certainly it is spectacular. Not all of the eighteen gemstone varieties are adapted to the potassium iodide – mercuric chloride process. The banded or fortification types do well, particularly coconut, Mexican fortification and crazy lace agates. Snakeskin, troyite and polka dot (white) agates become brilliant by this process – perhaps too much so. I am among those who consider the color as “gaudy”.

As I mentioned above, penetration in procedure B is very slow. If you mark out and proceed to grind and shape a cabochon from a slab colored by this process only to find that the color penetration is more or less incomplete, I suggest that you go ahead and completely finish the cabochon. Then subject it to both A and B of the procedure and the uncolored portion will become colored like the rest of it. Much shorter periods of time are needed in such cases because, on a finished cabochon, it is not necessary to have complete penetration.

I want to caution again against the extremely poisonous nature of mercuric chloride. However, it is safe to use if this is thoroughly understood and respected. Mercuric chloride has long been used in biological laboratories in very dilute solution (1:1000) in water to surface sterilize table and bench tops and other laboratory surfaces. I have never heard of anyone being poisoned thereby simply because the users observe the necessary precautions. Because of the potential hazard involved however, I was at first disinclined to include the potassium iodide – mercuric chloride process in this book but later decided to include it, on a “proceed at your own risk” basis. I am indebted to Professor June Roberts, Agricultural Engineer at Washington State University , for this process. Professor Roberts who is a long time friend of mine and fellow rockhound in turn got the suggestion of the process from Dr. George Austin, Professor of Chemistry at Washington State University.

Read also:

Gemstone Coloration and Dyeing – Table of Contents

By George W. Fischer
Copyright © George W. Fischer 1990
1961 Edition, published by Lapidary Journal. Inc. San Diego, California.
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