I was thumbing through some 1944 issues of The Desert Magazine when I stumbled upon descriptions of how agate was dyed red, blue, or green in Idar, Germany. Presumably this month-long process was likely the same one used to dye the prized agate marbles that were hand-cut in Idar-Oberstein.
I am posting these recipes here for novelty purposes, under the assumption that nobody is going to go hand-cut an agate marble and dye it using these recipes. If you do, please be very careful. And be sure to send me a photo of your finished product when you're done.
First, from the September 1944 issue we learn how agates are dyed red or "carnelian":
Everyone has observed how agates found on the surface and colored from brown to red are white inside when cut. This is caused by the heat of the sun. If such agates are further heated in ovens they often will turn red throughout due to the iron compounds in the stone. Agate, being crypto-crystalline, can have iron compounds introduced by absorption when little or no iron exists as determined by surface coloring. The best method is to soak the agate in iron nitrate. To make homemade iron nitrate a half pound of iron nails should be soaked in four times their weight of concentrated nitric acid. This will produce a slimy mass which should be allowed to settle. The solution is skimmed and the settling process repeated several times until the liquid is clear. Agates should be washed and freed of oil and dirt and immersed in the liquid for two to three weeks for stones up to about 1 1/2 inches thick and three to four weeks for stones up to four inches thick. Stones thicker than four inches rarely can be colored throughout.
After soaking the stone is saturated with iron nitrate which will become red when the stone is heated. It depends on your own ingenuity how this is done as the agate should be slowly heated and slowly cooled to prevent fracturing. People in the desert regions can use the sun but this is slow. People in California can store the stones in the bottom of their incinerators but in other sections of the country where these are not used probably the most satisfactory method is to bake the stones in a loaf of bread to permit slow heating and cooling. Those who have access to temperature controlled furnaces or laboratory equipment have the ideal arrangement.
All heated stones are brittle and present more difficult grinding and polishing problems as they easily fracture and chip.
Then in the October 1944 issue we learn about how agates are dyed blue:
Dissolve nine ounces of potassium ferro-cyanide in a quart of water and permit the stone to be colored to soak in it about three weeks. After washing it thoroughly it should be placed in a saturated solution of ferrous sulphate made by adding that chemical to a quart of water until no more will dissolve. In about ten days the stone is removed and dried in the sun. If the stone is not the desired shade it is returned to the ferrous sulphate solution until the desired shade is attained. Chalcedony and jasper colored in this manner is referred to as "Swiss lapis."
If a dark blue is desired a few drops of nitric acid and a few of sulphuric acid should be added to the ferrous sulphate or the first solution can be changed to ferri-cyanide instead of ferro-cyanide (red prussiate of potash instead of yellow).
The chemicals used in these coloring processes can be bought by the pound for very little from your local laboratory supply house or direct from chemical companies. Remember that sulphuric acid and water has an explosive action when warm and too concentrated. Otherwise no danger attends the use of these acids. All of these coloring agents work better if a way can be found to keep the solutions warm during the soaking.
And finally, green:
Make a quart of a saturated solution of potassium bi-chromate. A thin piece of agate should be soaked in this solution about two weeks, a half-inch stone about two months. After removal from the bath the stones are placed in a closed vessel containing lumps of ammonium carbonate and allowed to stay there about two weeks exposed to the volatile fumes. The stones then are removed and gradually but strongly heated until the desired shade of green is obtained.