This article is an excerpt from the book “Gemstone Coloration and Dyeing“, written by George W. Fischer, and discusses dendrites, moss, plume inclusions from the section ” Chemically Induced Inclusions”.
Dendrites, moss, plume and similar inclusions have added interest and value to gemstone for about as long as man has been aware of the beauty and gem potential of such “rock”. But apparently up to now, man has been dependent on inclusions formed in nature. The process by which they developed in nature has been only vaguely understood and thought to require long periods of time, even in the geological concept of time. Any means, therefore, of inducing the formation of inclusions in gemstone is automatically of more than transitory interest.
Quite by accident (as is often the case) I have been fortunate enough to discover a method of inducing the formation of dendrites, moss, plume, etc. in gemstone. It all started back in 1965. I had made for my wife a two stone bracelet, the snakeskin cabochons for which had been colored blue by the copper nitrate process (No. 1). After cementing the finished cabochons in place, I set the bracelet aside and, due to the distractions of more mundane demands, did not return to it to “clean it up” until a few weeks later. At that time, I noticed immediately that the two cabochons looked “different”; they were no longer the bright and uniform blue that they had been to begin with. A closer look precipitated an immediate and excited inspection with a magnifying glass. I could scarcely believe my eyes, for the magnifying glass confirmed what I thought I saw without it- dendrites! Nevertheless, there they were (and still are, twenty-five years later), just like some of the dendrites and moss sometimes seen in agate and chalcedony “in nature”, black and permeating through the entire depth of the cabochon.
My head swam with excitement, and I had to sit down and try to comprehend what had happened and what it might portend. I had made and mounted hundreds of cabochons of snakeskin agate before, variously colored by processes described in this book, and nothing like this had ever happened. Certainly snakeskin agate has no native dendrites or other such inclusions. So what was different? Where did the dendrites come from?
Finally, the explanation dawned on me from a basic principle I had first heard of in a high school chemistry course, and again in freshman college chemistry. The principle is “electrochemical displacement”.
The two historical cabochons had been made from a slabs of snakeskin agate thoroughly impregnated with a saturated solution of copper nitrate. The bracelet was gold plated, the plating probably over some form of iron. As lapidaries usually do when mounting cabochons, I had roughened the parts of the bracelet where the epoxy cement was to hold the cabochons in place. In so doing, I had scratched away the plating so that the underlying metal (probably iron) came in direct contact with the copper nitrate in the cabochons. The underlying iron displaced the copper from the copper nitrate in the agate to form free or elemental copper- in the form of dendrites in the agate . How does this phenomenon happen? I’ll attempt to explain.
The principle of electrochemical displacement can be likened unto a “pecking order” in the henhouse or to RHIP (Rank Has Its Privileges) in military circles. The chemical elements, particularly the metals, differ considerably in the extent to which they are “active”, that is, the extent to which they will react with water, acids, the salts of other metals and with other elements. Thus, the metals can be arranged in an “activities series”  in descending order of activity. The best known metals in this series in decreasing order of their comparative “activity” follow:
Potassium at the top of the list is so “active” that it will readily react violently with even cold water, liberating hydrogen from the water and creating considerable heat. On the other hand, gold at the bottom of the list is so inactive, or “inert” that not even strong acids will affect it.  Platinum and silver are nearly as durable and it is this durability even more than their beauty that make them “precious metals”.
Of immediate concern to us relative to chemically induced inclusions is the fact that, in general, any metal in the above list can be displaced from its compounds by any other metal above it in the list, but in turn, it can any other metal below it. Thus, iron is above copper in this “pecking order” and can displace copper. And in the case of the accidentally induced dendrites described above, that is exactly what happened. The principle of electrochemical displacement can be further illustrated by the fact that a nail when dipped in a solution of copper nitrate (or other copper salt) quickly becomes copper coated or “plated”. The iron in the nail displaces the copper in the copper nitrate solution. The copper goes out of the solution and is deposited as free copper plated over the exposed surface of the nail. So in the case of the historic (to me) accidental production of dendrites in the two bracelet cabochons when I scratched through the gold plating on the bracelet in order to create a roughened surface for better adhesion of the cement, I exposed the underlying metal (steel). When this base metal came in contact with copper nitrate, the copper was displaced from solution in the agate, causing the release and deposition of free, elemental copper. Fortunately, this displacement took place not merely on the surface, but penetrated deep into the interior following the porosity of the agate.
While the above theory, superficially explained here and perhaps oversimplified, seemed plausible enough to account for the mysterious formation of the dendrites, it did not answer the questions: Why had it not happened before? Then I remembered that these two particular cabochons had been made and promptly mounted from a freshly soaked (in copper nitrate solution) slab of the snakeskin agate. Thus the slab had not had a chance to dry out, and the pores still contained a solution of copper nitrate. Apparently, too, I did not get a complete surface application of the epoxy between the cabochon and metal so that in each cabochon there was a direct contact with the copper.
My wife never did get the bracelet (not that one). She was quite willing to sacrifice it to the cause of “science”, applied to the lapidary arts. I supposed at first that the base metal of the bracelet might have some particular potency as a displacent metal to form inclusions in the agate. So I dismembered the bracelet and fragmented the bezel portions into many small pieces, each one of which I hoped would make more dendrites. They did, but I soon discovered that I need not have sacrificed the bracelet. Any old form of iron would do the trick: nails, tacks, wire, paper clips, nuts and bolts, etc. And I soon learned that, as prescribed by theory, other metals besides iron above copper in the activity series would likewise induce the formation of dendrites and other inclusions.
Also by theory, other compounds of copper in solution should likewise be useful to induce the formation of inclusions. So I soaked slabs of snakeskin agate in saturated solutions of copper acetate, copper sulfate and copper chloride. The first two proved to be useless for the purpose (not sufficiently soluble?), but the chloride proved to be far superior to copper nitrate. It responds more readily, more reliably and with more attractive inclusions. Therefore, as far as copper is concerned, all further experimentation was confined to copper chloride.
The principle of electrochemical displacement suggested that other metals should be amenable to this method of inducing inclusions. To test this out, I have tried silver (nitrate), tin (chloride and sulfate), lead (chloride), nickel (nitrate and chloride), cobalt (chloride) iron (chloride), cadmium (chloride), chromium (chloride) and zinc (chloride). Slabs of snakeskin agate were soaked in solutions of salts of these metals and then small bits of metals in the activity series above those in solution in the agate were placed in contact with the slabs. To make a long story short, positive results were obtained only with silver nitrate and tin chloride.
Only magnesium and aluminum would displace silver out of silver nitrate to form inclusions of metallic silver in the agate. This had tremendous possibilities, except that the entire slabs turned progressively darker until the dendrites were completely obscured. Apparently through the agency of light, the silver nitrate in the slabs was reduced to minute silver particles by traces of organic matter in the water.
On the slabs soaked in tin chloride, only aluminum and zinc would displace the tin from solution in the slabs to form tin inclusions. Tin admittedly does not have the glamour of silver, but since the tin inclusions look like silver and since the slabs in which silver inclusions were induced turned intolerably dark,
further experimentation with silver inclusions was dropped in favor of tin. In fact, all further experimentation on chemically induced inclusions in agate have been confined to refinements, improvements and ramifications of the two process that induce copper and tin inclusions respectively. At the time of present writing, the search for such refinements, etc. is still continuing, but sufficient progress has been made in developing stable processes that it seems feasible to include them in this book. I hope it will stimulate other rockhounds to do, some experimenting of their own.
- Also known as “electrochemical series”. Silver Platinum Gold
- However, a mixture of nitric and hydrochloric acids known as “aqua regia” will dissolve gold nitrate in the cabochon and the base metal of the bracelet with no cement in between.