Fire Agate: A Cutters Guide


Copyright October, 2013
Author’s Note: A few paragraphs below is a copyrighted article, FIRE AGATE: A CUTTER’S GUIDE that appeared in the on-line Eclectic Lapidary, September 1, 1998 - Volume 2, Issue 9.
Its words are my property and I’m exercising ownership by publishing them here in full, with new images and some revisions to bring it up to date as of October 2013.  I’ve recognized my exact words or very thinly-disguised rewrites in other articles by several others. That’s okay. While it would have been more ethical and satisfactory to have received credit for my original work, my article was written for the purpose of education and experience-sharing. The plagiarizers know who they are and what they’ve done.
My original article left out a discussion of fire agate’s history and its role in the gem world. I’ll attempt to rectify that oversight in this foreword. 
In my opinion fire agate still is one of the world’s major “undiscovered” gem values. The best stones have amazing play of color and brilliance similar to fine Australian black opal but are many times harder and more durable. Unlike opal they are even suitable for use in men’s rings. But current fire agate prices are drastically lower than black opal and represent a great bargain in my view. 
Fine fire agates are truly rare. Research indicates this material is currently found at only a few locations in the Sonoran Desert in the southwest U.S. and Mexico.   It seems logical they should be found elsewhere but no important new locations have been announced in the many years since the gem was first recognized in the 1940s.
Places where fire agate is found in the Sonoran Desert -- California, Arizona, New Mexico and Mexico -- were once volcanically active with many hot geysers and geothermal springs, much like today’s Yellowstone Park. The silica that makes up fire agate was carried by hot water from below the surface and deposited in layers in a hodge-podge of miniature stalagmites. Iron minerals like limonite and goethite were deposited between the layers of silica and they are the source of dramatic and brilliant colored iridescence.
Its great rarity is one of the reasons fire agate isn’t more popular. Most stones also require custom-made mountings and many jewelers simply don’t want to bother – they want standard-sized stones to fit into standard-sized settings to easily serve a mass market. Fire agate is definitely not a mass market gem! Each stone is unique and must be appreciated and treated with the care and individuality it deserves.
Another marketing obstacle relates to the stone’s rather uninspired name. The gem world tends to consign all agates no matter how beautiful and rare to the amateur enthusiast’s realm and almost never thinks of them as fine gems. So “fire agate” suffers the widespread prejudice against agates in general.  It, along with many other really fine and rare agates, deserve much more.
It should be noted here that most “fire agate” really is not agate at all but a variety of reddish-brown chalcedony called Sard. It mostly lacks the banding that distinguishes agate from chalcedony. But the fire agate name is in wide use despite a marketing effort some years back to re-name it “Precious Sard” so the “agate” usage seems firmly established.     
Somehow, fire agate has also gained a reputation as a “man’s gem,” and is frequently set in massive gold gents’ rings that don’t have wide style appeal. That’s unfortunate because it serves as very fine ladies’ jewelry as well. For some inexplicable reason it’s also featured in Southwest-style Indian jewelry, probably because it’s found in the region and was used by local silversmiths due to its availability. But it’s a gem that is adaptable to many jewelry styles and it should have a much wider appeal.
As for written material on cutting fire agate, as mentioned below, gem carving has since gained enormously in popularity and many of books and articles have been written about it. Many of the techniques so described can be applied directly to cutting fire agate. I’m adding a new section on fire agate carving at the end of the original article.
Fire Agate: A Cutter's Guide
by Richard O. Martin
Copyright September, 1998
When I first decided to try to cut fire agate a number of years ago I searched for books and articles on the subject. My search turned up very little. A fair amount has been written about fire agate collecting locations but I haven't found much on its lapidary treatment. That's too bad because this material can be downright intimidating to the would-be cutter.
Having seen some beautiful cut gems at a gem show I ordered some rough. When the postman delivered the package a couple of weeks later I eagerly tore it open and was utterly dismayed by what was inside. I don't know what I was expecting, maybe easily-sawed 38 x 52 mm cabs glowing on the surface, but what I received was quite different.
"Scrambled quartz" was the first thing that crossed my mind. After studying the rough carefully with my Optivisor I began to suspect that this was a gemstone designed by the same guy who invented fractures, undercutting and saw sludge.
Looking back on my many problems learning to work fire agate by trial and error, I decided to write this article. It certainly won't tell you all you need to know; that knowledge comes only from experience. Besides, I don't KNOW everything there is to know. But using the following methods I've cut hundreds of beautiful fire agate cabs, and you can too.

Fire agate has been compared to a layer cake with frosting between each layer. The "cake" is agate that ranges in color from clear to dark brown and the "frosting" consists of layers of the iron oxide minerals limonite and goethite that experts say causes white light to separate into spectral colors, creating its "fire" or iridescence. There is often a "cap" of milky-to-translucent chalcedony, usually in the form of a "rose," that marks the stone's top. Characteristics of rough fire agate can vary considerably, depending on its source.
In an ideal world, the iridescent layers would be as flat as the frosting on a real cake. But, alas, fire agate is deposited in botryoidal form, meaning the structure resembles the form of a bunch of grapes. A lot of people simplify this terminology by calling it "bubbly," which also describes it pretty well. Whichever word you prefer, most hunks of fire agate rough are twisted and gnarled with the "cake" and "frosting" deposited at all sorts of crazy angles. This creates a great challenge for would-be cutters because the sought-after color layers follow the highly irregular contours of the underlying brown agate.
The art of cutting this material lies in finding the brightest, best-colored layer and following its contours wherever they lead. The object is to obtain the largest, brightest stone possible, while never cutting through the iridescent layers. Sometimes a cutting slip reveals an even better color layer, but more often, the stone is ruined.

The "bubbles" in fire agate vary tremendously in size, from almost microscopic to singles yielding cut stones in the 18 x 13 mm range or larger, in some cases a lot larger. Generally speaking, these larger ones are best suited for cutting on standard lapidary equipment, because the shapes are easier to work on flat grinding and sanding surfaces.
Fire agate with intricately contoured, irregular or “warty” botryoidal color planes can only be fashioned adequately using gem carving techniques. This is exacting work, accomplished with diamond points and compounds in conjunction with specialized carving machinery. These techniques will be generally addressed later on.  
But don't be frightened away from trying this exciting material because you're not a carver. Standard cabochon-cutting equipment is entirely suitable for cutting many types of fire agate. Just be careful in selecting rough and take the time to carefully study how your stones were laid down by nature.

I'll take a moment here to discuss rough fire agate vs. pre-windowed pieces. I generally prefer to cut from selected rough but occasionally superb windowed material is available at reasonable prices. Naturally you pay a premium for someone else's knowledge and labor in doing the "dirty work" for you. Now and then, you can find some really great stones in this form but I've learned that pre-windowed often means pre-ruined. The best fire layers are often cut through, leaving only small stones to be salvaged from what remains. But occasionally there are even better fire layers below the ruined ones, so be a careful shopper.
The gemmy layers are of varying thickness. The brightest and best layer is often just at the junction of the chalcedony "cap" and the brown agate. Sometimes these layers are only a thousandth of an inch or so thick, and sometimes they are best cut by leaving a slight amount of clear chalcedony over them as protection if possible. If one sands right down to the color plane, subsequent sanding and polishing can often destroy it although sometimes it can often reveal the best and brightest colors. Learning how much to sand is an important part of the fire agate experience.

The trick is deciding which layer is the brightest and most desirable and making it the top of your stone. Fire agate has the maddening habit of being most colorful in exactly the places where it's structurally impossible to cut a cab. But that also seems to be true of most of the other materials I've cut over the years, especially opal!
You can usually expect the color layers to run in succession from bronze to gold to red and then to green. If you're exceptionally lucky, the green will be followed by the very desirable purple. And if you've really been living right, you will find very rare blue highlights. Some say that a stone showing four colors at once can be called a "Peacock," a term that I believe was originated by the late fire agate miner/author Warren Hughes, who published several articles about it in Rock & Gem Magazine.

Occasional rough pieces have a layer of white or yellowish opaque common opal on what I call the bottom. My experience tells me these "opal bottoms" have the potential for unusually bright interior colors. Some are duds and not all high-grade stones have the opal. But I've learned to give these pieces special treatment.

Now to the cutting. Wet and inspect your rough under magnification in bright sunlight. Look it over as closely as a banker checking out a loan applicant. If you can see through the chalcedony, try to find flashes that indicate color layers. Don't be discouraged if you don't see any; the color may still be there but you'll have to work harder for it. Next, carefully examine the edges of the brown agate at various angles in the light, searching for color. Color planes may be revealed at several levels within the agate and give indications of their thickness, brightness, colors and general orientation.

Some careful trim-sawing can save a lot of grinding. But never cut into the dark agate layer! "Candle" the chalcedony against a bright light source to judge how deep to saw and always leave more rough stock than you think you should.
Removing this "cap" is hard work, tough on equipment and sometimes fruitless: lots of good-looking rough turns out to be your basic good old throwing rock. But there's no way to tell for sure until enough of the cap material has been reduced to allow any color underneath to be seen.
Overzealous trim-sawing and the wrong kind of light have ruined more of my stones than any other cause. Really THINK before you make a saw cut. Being able to really see what you're doing is the single most important factor in cutting this stone. Maybe your eyes or techniques are better than mine, but I now cut all my fire agate in direct sunlight. I made a portable table with casters on the legs and on warm, sunny days when I have fire agate to cut I slather on sunscreen, roll the unit into the driveway and have at it.

Direct sunshine makes it possible to do some very accurate grinding. It's the nature of most fire agate cabs to require contouring to follow the color layer, and flat grinding wheels are designed to cut convex or flat surfaces. One of the toughest jobs is to grind into the chalcedony-filled "valleys" between the larger individual bubbles if you decide not to saw them apart.  
Specialized diamond wheels shaped for this job work well but are expensive. I use one edge of the grinding wheel to grind one "hillside" first, then switch to the other edge for the opposite one. With care, the face of the wheel does all the work and the edge gets very little wear. 

WARNING: This type of cutting can be dangerous. The wheel can "grab" the piece of rough (and your fingers, too). Work very cautiously, bearing in mind that pressure on the edges of most standard diamond wheels can quickly ruin them. Use common sense, work carefully, and never use excessive pressure.  [NOTE:  Sintered diamond wheels, not generally available when this article was written, cut from the edge as well as the face and are not damaged by this type of grinding.  There are also specialized plated metal grinding wheels that have diamond coatings on both edge and face.]
I use metal-bonded diamond grinding wheels and have no experience using silicon carbide wheels on fire agate. Some cutters have told me they achieve good results with them because they can be shaped for special purposes. Use caution and never use wheels that are out of round or waterlogged on one side.

When grinding has proceeded far enough that vague brown shapes can be seen through the chalcedony, any iridescence should be visible. Before there's any real danger of grinding into it, I move from the grinding wheel to an expandable rubber sanding drum fitted with a 100-grit silicon carbide belt for wet-sanding. Instead of aligning the sanding belt evenly with the outer edge of the drum, I pull it about 3/8-inch past the edge (I generally work on the left side of the drum, but either side is fine).
After you start the drum rotating, this fold-over "flap" provides an arrangement for contour-sanding I've found works very well if used with enough water. (A new belt will remove material rapidly, so allow for this. When the "flap" becomes too worn simply tear away the part that's worn, starting the tear with a sharp knife or scissors, and readjust the belt so fresher grit is exposed.)

Use the edge of the sander to smooth out crevices and concave areas, and its face to work larger, flatter portions of the stone. You'll quickly discover that frequent sanding is the key to seeing where you're going in the grinding process. Grind a little, sand and inspect carefully in good light. Repeat this procedure of alternate grinding, sanding and inspection until the desired color is fully revealed. The color layers are thicker on top of the bubbles and thin out on the slopes and edges, so do most of your initial sanding on the tops.
I learned long ago that fire agate, like opal, tells the lapidary how it should be cut. I don't try to force it into any particular shape except to make sure it has pleasing lines. I do my own wax work and casting, and find free-form shapes pleasant to design around and work with. If you are uncomfortable with anything but symmetrical shapes or intend to cut only calibrated sizes for standard mountings, you may find fire agate frustrating. It just doesn't adapt easily to this style of cutting unless you're resigned to wasting a lot of colorful yield. I believe in letting the patch of brightest color define the edges of the finished stone. To me, the ideal fire agate cab is a complete bubble that shows off the material's inherent sinuosity. It provides an uninterrupted flow of bright color from the crown to where the metal of the setting begins.

Don't be in too much of a hurry to grind a final bottom on the cab. The excess material makes a good “handle” for holding the stone while sanding. I wait until all the color is uncovered, then grind the bottom to orient the best color on top to avoid "directional" color display. Look directly down at the top of the stone and move it around in light coming from directly above your head. When it has the best orientation of color as you rotate the stone through 360-degrees, use a Sharpie pen or an aluminum scriber to mark the future girdle and then grind the bottom flat to your marks.
Remember: in most cases, the battle to reveal fire agate's hidden beauty is won with the sander, not the grinding wheel. As with opal, inexperienced cutters ruin fire agate because they're in too much of a hurry. A heavy hand on the grinder or trim saw, coupled with a “hurry up” attitude, will almost guarantee failure with either stone.

I mentioned earlier that one shouldn't grind or sand too much on ultra-thin color planes for fear of cutting the color away. That's absolutely true. But the term "thin" is relative. Color layers that may look impossibly thin to a novice cutter may seem delectably "meaty" to one with more experience. And this brings me to one of the most valuable cutting techniques I've learned: playing the game of "chicken" on the sander.
Adjacent layers of iridescence exhibit different colors. They also can exhibit enormous differences in brightness. Before grinding the outline to finished shape, I usually spend a lot of time sanding the stone to perfection. Often, stones that showed only "Ho-hum" color and brightness in the bronze and gold layers have been turned into extremely brilliant and valuable Peacocks by my having the patience -- and courage -- to stick with the sanding. There is no feeling in the world like having a drab bronze stone suddenly start showing dazzling green -- then purple -- then blue, all mixed together in glorious highlights along with reds, yellows and oranges. Wow!

Sometimes you win, more often you lose. But if you have reason to suspect that multiple or brighter color layers are present it's well worth the gamble. Nature stacks the deck with a lot more drab stones than Peacocks, and you'll end up with more "ho-hum" stones than you ever wanted anyway. Go for the gold -- and the red and the green and the purple!

After being used for a time, the 100-grit sanding belt produces a fairly good pre-polish while still cutting rapidly in some areas (you'll find you can use different sections for different effects). It's the only grit I use in final shaping and pre-polishing. After the top of the stone is thoroughly sanded on the most worn portion of the belt, it can be polished in your usual way.

Even though I've been working with this material for a lot of years now, I'm far from having all the answers. But perhaps this article will help a few more lapidaries [ists] get started cutting a stone that can be as rewarding as it can be frustrating. I invite correspondence with other cutters to exchange special tricks and techniques with this stone. Meanwhile, good luck!