Fundamentals of Lapidary Part 4, Cabochon Cutting

by Donald Clark CSM

Cabbing is the most popular form of gem cutting. It requires a fair amount of skill, but is something almost everyone can master. Rough material for cutting can be found or purchased inexpensively. Many people never buy their rough and are content to cut the many stones they find or trade for. As one gains skill, you can move into more desirable materials like turquoise and lapis lazuli. If you wish, you can graduate to highly valuable materials like cats eye chrysoberyl and black opal.

Tools

The tools used for cabbing can be as inexpensive or elaborate as your taste runs. The most popular cabbing machine is the Genie by Diamond Pacific. It has six, permanently mounted wheels. The first two are 100 and 260 grit diamond on solid wheels. The other four range from 280 to 14,000 grit. They have a foam backing, which conforms to the curvature of the stones. There are two moveable “geysers” which supply water to the wheels for lubrication. You cannot polish everything with one method, so there is a flat disk that can be mounted on the end. This disk can hold a variety of pads and polishing compounds; enough to be able to polish anything that you might encounter.

The advantages of the Genie are that you never spend time changing accessories and that the diamond wheels last almost forever. Its primary disadvantage is cost. At approximately $1500, this is a major investment. It is also large and heavy, which is fine for a permanent setup. However, if you need portability, this is not the right unit for you.

Going down in cost, Loretone offers a unit for approximately $500 that has a built in saw, a silicon carbide grinding wheel, an expanding drum and a flat disk for polishing. The quality of the machine and the work you can do with it are both excellent. The cost reduction comes in the wheels.

Silicon carbide wheels round in the center. They soon become difficult to use and eventually unusable. To flatten them you need a diamond tool that costs about $30. Nor do they do not last nearly as long as a diamond wheel. A good six-inch diamond wheel will cost you between one and two hundred dollars, where an equivalent silicon carbide wheel will run about $30. While the initial cost of silicon carbide is much lower than an equivalent diamond wheel, in the end the diamond is less expensive. You should bear this in mind when replacing a wheel, or building your own machine.

The Loretone also uses an expanding drum that is four inches wide. To use it, you place a belt around the drum and, as the machine comes up to speed, the drum expands and holds it in place. There is a wide variety of belts available. They come in silicon carbide or diamond and in grits from 100 to 50,000…. In the long run, these belts cost more than a wheel with imbedded diamond, but the difference is not as significant as with the silicon carbide grinding wheels. The primary disadvantage is the time it takes to change belts between grits. The machine needs to come to a complete stop; remove the old belt, replace it with a new one; then bring the machine back up to speed. To some people this will be insignificant, to others just a minor disadvantage. You need to look at your needs to determine how much this will effect your enjoyment.

On the very bottom of the price scale is the Rock Rascal. Its cost is just $240 if you add your own motor. It only has one station, so you are constantly changing accessories, and it will only hold one-inch wide wheels. However, it has been on the market for several decades, which speaks volumes for its durability.

There are a number of other machines on the market with a variety of features and prices. Many people have made their own machines. The requirements are an axle of the proper diameter for the wheels you chose; a motor; a pulley system to attain the proper speed and a method to keep the wheels wet. This is usually a drip system. Another simple system is a sponge that is arranged to sit in water and constantly wipe the wheel clean.

It should be pointed out that you can cab on a faceting machine, but you cannot facet on a cabbing machine. Bear this in mind if you are a beginning lapidary and budgeting your equipment costs.

You can shape your cabs with the same coarse laps you use for faceting. Clean your coarse lap, then put a piece of firm, ΒΌ” thick, foam rubber on top of it. This will hold pieces of 600 wet/dry sandpaper, (that you have to cut to shape,) for smoothing. To polish, replace the sand paper with an Ultra Lap, a piece of leather, or other polishing pad charged with your favorite compound.

Techniques

In this section we will describe how to cut an oval cabochon. This is the most common shape for cabs and the other shapes just require a minor adjustment of technique.

Most cutting begins with a slab. Yes, that means you will need a saw. Some pieces can be cut without slabbing, but most will need to be cut into 1/4″ to 3/8″ thick slices. (See our article, “Making Smaller Pieces.”)

Begin by placing a template over the slab. If your material is a solid color, you are just looking for the largest piece you can cut. If it has a pattern, then you are looking for the shape and size with the most eye appeal. This is a bit of a guessing game, as the pattern will change as you round the top. Look at both sides of the slab to get an idea of how it will change.

It isn’t always necessary to use a template. Some valuable materials, most notably opal, are usually cut freeform for maximum weight retention. However, on most materials you will want to end up with a calibrated size and shape. That is because they fit into premade settings. It isn’t cost effective to save a little weight on most cabs and then have to spend a dozen hours, or hundreds of dollars, to make a setting for it.

When you have chosen your area, mark it for cutting. An aluminum pen is ideal for this purpose. It will make a mark that will not wash away, as a pencil will. Many materials will absorb liquid ink, which leaves an unsightly mess that has to be cut away.

Now that you have your cab outlined, take it to the saw and cut away as much excess material as possible. Until you get used to this process, it is helpful to draw guidelines with the aluminum pen and a ruler.

If you are working with a large gem, you can move on to the grinding stage. Smaller stones should be dopped to save your fingers from unnecessary abuse. (See the article on “Dopping Techniques.”)

Shaping

Now it is time to begin cutting. If working with something hard, like a piece of agate, begin with your coarsest wheel. If you are working with something soft, like opal or turquoise, or something small, begin with one of the smoother wheels. If it cuts too slowly, you can always go to a coarser one, but caution dictates that you start gently.

Bring your machine up to speed and get your chosen wheel thoroughly wet before starting to cut. The water serves two important purposes. It acts as a lubricant, keeping the friction and heat down. It also carries away the swarf, (the cutting debris,) which prevents the wheel from clogging up. Most manufacturers recommend that you use an additive for additional lubrication. Unless your machine is made entirely of stainless steel, you will need an additive with rust preventative properties as well.

When grinding, it is important to use the lower quarter of the wheel. If you try to work above the centerline, there is a good change the gem will be pulled out of your hands. “Pulled” is tame compared to what actually happens. The gem is yanked violently form your hands and flung to the far reaches of your shop. If you can find it again, it may well be broken.

Begin by grinding the excess material down towards the outline you have drawn. Use a pair of calipers to measure your progress. Templates are great for shapes, but are not accurate for dimensions. Leave yourself some room for the remaining steps. Depending on the size of your cab, you may want to leave a half-millimeter or so. This is something you will learn from experience, as the amount varies with the hardness of the material and the size of the gem.

Once you have the outline shaped, it is time to begin the process known as “Pealing the Apple.” Grind a bevel all the way around the gem at about 45 degrees, bringing it 2/3 to 3/4 of the way down to the bottom. Then grind another bevel, again all the way around the stone, at about 60 degrees, leaving some of the previous cut showing. Keep pealing the apple, cutting bevels at increasingly steeper angles, until they meet in the center.

This is the area that causes the beginner the most trouble. It seems that just three cuts will completely cover the stone, except for maybe a tiny area right in the center. That is not good enough. If you have even a tiny area that is flat and not properly domed, it will not take a proper polish. To make matters worse, it is right on the very top where it shows the most!

You have to be patience and learn to get the entire stone evenly domed. Look at the stone from both ends and both sides. The curvature should be even in every direction and all the way to the center. You should make this inspection at every step, but especially in the first, coarsest stage. Any irregularities are easier to correct now than later with the finer grits.

Now we are going to get ahead of ourselves for a bit, just so you know where we are headed. As you are shaping the gem, leave a small, vertical area on the sides before it begins to taper in. You will also want to sand a very small bevel on the lower edge of your girdle. This is done in the fine sanding stage. The bevel prevents the edge from chipping and it also leaves a small area for solder.

Cabs are usually placed in a bezel setting. The bezel, a thin strip of metal, is soldered to the main piece. Hence, you need a little room for the solder. To hold the gem securely, it needs to be bent inward, over the curvature of the stone. The vertical area on the side offers support, without having to bend the bezel too far in. When finished, you want to see mostly the stone. You do not want the bezel to come so far over the edges that it distracts from the gem.

These steps are particularly important with brittle opal. If you leave a sharp edge on the girdle, the stone can chip or crack when tightening the bezel. The steeper the sides, more support the gem receives. Of course this can be over done. The above illustration shows ideal proportions, from the metal smith’s point of view.

Smoothing

Once you are satisfied with the shape of your gem, it is time to start sanding out the coarse scratches. If you are using diamond, this will be in a series of steps. Typically, you would use a combination like 260, 600, and 1200. Inspect your gem after each step. Look for proper curvature, as you did before. Also, check to make sure you have removed all the coarse scratches from the previous step. This is very important. If you don’t get them all out before moving on to the next step, you will have to either back up and do it again, or settle for a low quality finish.

The gem needs to be dry for you to see the progress. This can be challenging in a humid environment. If possible, warm the room before starting. If you can’t warm the room, at least use warm water. Have plenty of towels available and dry your hands before trying to dry the stone.

If you are using silicon carbide, all you need is 600 grit. It wears fast and becomes the finer grit you need for a prepolish. Unfortunately, these have limited uses. You may have to have two 600 belts in use; one for shaping and a well used one for prepolishing.

The quality of your prepolish, the final fine sanding, cannot be overstated. This is the single most important element in getting a high polish. The surface should be ultra smooth, with no visible scratches at all. Any time you find you are having trouble getting a high polish, go back to the prepolish step. Soft materials like lapis and opal scratch readily with diamond. 1200 diamond is a fine prepolish for agate, but you might need something much finer, like 3,000 or 8,000 for softer gems.

Polishing

There are hundreds of polishing methods, which can be very confusing to a beginner. If you buy a new machine, it will probably come with a polishing system. That is a good place to start.

An old lapidary trick is to use aluminum oxide on suede, soft side out. This is the most versatile polishing method available. It will put a high polish on more materials than any other method.

Quartz responds best to cerium oxide, which does well for opal too. You have to be very careful about heat build up with opal. Stones get hot quickly while polishing and heat can destroy opal.

Jade, on the other hand, needs a lot of heat and friction to polish. Use chromium oxide on leather or felt.

Diamond can be used on leather or specially made pads. However, it has never gained the popularity with cabbers that it has with faceting. It is more expensive than aluminum oxide and doesn’t offer any significant improvements, except on ruby and sapphire.

These are the most common and most useful polishing methods. With them, you will be able to get a mirror polish on any material.

That mirror polish is important. It is a fair amount of work to come up with a machine, learn how to use it, find the rough, orient it and finally cut a gem. When you get that mirror high polish on a beautiful stone, it all becomes worthwhile.

Judging Your Work

When examining a cabochon, begin by judging the polish. Look for any scratches or pitting that will reduce the amount of light reflected from the surface.

The next thing to look for is how even the contour is. A cabochon should have an even curvature to its surface. Look at the cab from both ends and both sides. The shape, (the curvature,) should be a mirror image from side to side. No area should be thicker than its opposite and there should be no bulging.

The second way to judge the shape is to hold the gem so light reflects off its surface. Move the gem so the light travels across the top. If the surface is properly cut, you will see the band of reflected light glide evenly over its surface. The band of light will begin to snake if there are any irregularities.

The very top of the gem is where you are most likely to see a problem. Often a small area will be somewhat flattened. This is hard to see when viewing from the side, but obvious as light passes over it. The fact that light doesn’t flow smoothly over this area is why it is considered to be second-rate workmanship. However, if you look closely, that area probably doesn’t have as good a polish either.

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