Cutting Boulder Opal

(The article below is an updated version of a two-part article that I wrote and appeared in Lapidary Journal in March and April 1998.)

This describes the way I cut boulder opal. I do not claim that this is the only method or the best method. It is a method that I have developed over the past 10 or so years from the suggestions of Australian miners and cutters as well as trial and error. I have had some very beautiful results. I am going to be describing cabbing boulder opal with a machine using diamond wheels. I use a slightly adapted Diamond Pacific machine, which has diamond wheels with grits of: 220, 280, 600, 1200, 3000 and 8000. I also have a second, smaller, cabbing machine which has an 80-grit wheel. I will not be discussing using a flexible shaft machine. While much boulder, particularly much of the high-grade material, requires the use of a flexible shaft machine, in many cases very attractive stones can be successfully cut using diamond wheels.


Buying boulder opal rough is very risky. I do not make any promises, but I will just make a few suggestions.

If you are looking at rough at a show talk to the dealers. Ask them for suggestions about finding some inexpensive rough that will be relatively easy to cut. Make sure that the ironstone around the opal is fairly sturdy. Beware of deals that sound too good to be true. Buy rather low-grade material to start with. Cabbing lower grade material is an inexpensive way to get to know the characteristics of boulder opal as well as the characteristics of the dealer. You might not end up with a beautiful stone right away, but you will improve your technical skill so that you will be able to cut that beautiful stone in the next parcel of rough.

Look for a piece with a layer of opal that is as flat and consistently shaped as possible, but do not necessarily expect much color. Look at the grain and pattern of the ironstone to give you clues about the direction that the seam will run. When opal occurs in ironstone that shows fairly flat, straight, parallel lines in the matrix, then there is a good chance that the opal will follow these lines and also be fairly flat. If the ironstone displays wavy lines then there is a good chance that the opal will also be rather wavy.

As with any opal, look out for cracks. Avoid considering rough that has been soaking in oil or glycerin. This can hide cracks. If the rough is wet, let it dry while you are talking to the dealer, then look for serious cracks. Some pieces of rough may be partially faced (the ironstone has been ground down to expose the opal layer). This can sometimes give you a better idea of what you are buying and save you some work, but ask yourself why the dealer is selling this and not cutting it. Look out for faced material that has very deep gashes in the opal layer.


Put on your old clothes. Cutting boulder opal usually splashes around quite a bit of brown mud, which stains. You should select a well-behaved piece of rough to start working on. If necessary, saw away the excess ironstone so that the remaining piece of rough can be managed with a trim-saw.

I look at whatever line of opal is visible and at the pattern in the grain of the ironstone in the piece of rough. I try to estimate the direction that the seam might run. Using a combination of a trim-saw and an 80 or 220-grit wheel, I try to expose the edges of the line of color. When using the saw try not to get too close to the line of opal. The seam of opal can make some unexpected turns that can be ruined with the saw blade. On more than one occasion I have gotten too close and put the saw blade through part of the seam of opal.

I go back and forth between the wheels, cleaning the mud off the stone often, and looking for signs of opal. As soon as any sign of opal is exposed I stop and turn the rough to find the other edges of the opal. I try to make sure that the 80-grit wheel does not touch the opal. It can be very damaging. Once I have found what I think are the edges of this line of opal, I look carefully at the shape of the line and the nature of the ironstone on either side of the line to try to determine how to orient the stone and from what direction I should attack. 


Deciding which side is going to become the top of the stone is based on a combination of many factors. The main things to consider are the shape of the seam of opal, the coloring of the surrounding material, and the strength and thickness of the surrounding material.

If the line of opal is curved it will usually be easier to cut if the top of the stone is the convex (outward curving) side and the bottom of the stone is the concave (inward curving) side.

It usually is desirable to have the material that is immediately below the line of opal to be as dark as possible. This dark background will help to make the colors of the opal stand out. Occasionally there will be a thin layer of black potch between the line of opal and the ironstone. Try to cut the stone so that this black layer will provide the background for your stone. This type of stone is called a boulder black or a black boulder opal. Boulder blacks can be quite brilliant.

It also is important that the boulder opal has a solid base. If the ironstone on one side of the line of opal is weak or too thin then it will not provide the strength that is necessary to be the base of your stone.


I generally start with the 80 or 220-grit wheel to remove the excess ironstone that is covering the top of the opal. I only use the 80-grit wheel if the ironstone layer is more than about 3mm thick. When using the 80-grit wheel be careful and use light pressure. The 80 grit wheel can easily cause chipping or make deep grooves if it comes into contact with opal.

When I am removing this ironstone layer I try to be very careful and very observant and use quite a bit of water. I stop and wipe off the surface of the stone many times to see how close I am to the opal or if any opal has been exposed. The seam of opal can take surprising turns or bulge in unexpected areas. You do not want to grind away what could have been a good surprise.

As I get closer to where I think the opal is I use the 220 and then the 280 grit wheel. I try not to come into contact with the opal when using the 220-grit wheel. The 220 wheel can very quickly remove quite a bit of colorful opal or introduce serious gouges. I use lighter and lighter pressure with the 220 wheel as I get closer to the opal. I try to make sure that the 220 wheel does not touch any opal.

I usually continue to remove ironstone using the 280 wheel until I just barely expose opal in one area of the surface or until I am rather confident that I am very close to the opal layer. Sometimes, but not always, just before you reach the opal, the coloring of the ironstone will change.

Then I switch to the 600-grit wheel to try to expose more of the opal. At this stage I am not trying to remove all of the ironstone from the surface. I usually go back and forth between the 600 wheel, with medium pressure, and the 280 wheel, with light pressure, to remove enough ironstone to give me a good idea of where the opal is, what its coloring and quality is, and in what direction the layer of opal runs. Once I have this information, I stop working on the top of the stone and look at it thinking about what shape it would naturally tend to be.

At this point you might have the surface of the opal very clean and have a very clear idea of the shape of the opal, or things might not be so clear. If you think that you have a good idea of the shape of the opal, use light pressure on the 220 wheel or medium pressure on the 280 wheel to shape the edge of the stone or to remove any irregularities from the edge of the stone. This shape will not necessarily be the final shape of the stone, but it will probably make the stone easier to work once it is on a dop stick.


Using the 220 wheel, or gently using the 80 wheel, I would prepare the stone for dopping by flattening its back. I also use this stage to reduce the thickness of the back of the stone so that it is just slightly thicker than what I think would be appropriate for the finished stone. If necessary, I can remove more at a later time. I start by gently placing the back of the stone against the 80 or 220-grit wheel while rotating the stone in my fingers. If you use the 80-grit wheel, use light pressure and be very careful.

Remember you are flattening the back not cabbing it. The ironstone backs of most of my stones are probably no more than 5mm thick, unless the stone is quite large or has an unusual shape. Be careful because the nature of ironstone varies quite a bit. Some ironstone can be quite strong and consistent, and some can be rather weak and have sandy pockets. Inconsistencies in the ironstone can cause the stone to be unexpectedly grabbed by the wheel and sent flying. Make sure that the back is thick enough to be strong enough to support your opal.

Once the back has been flattened on the 220-grit wheel and any extreme irregularities have been removed the stone is ready to be dopped. Before dopping make sure that the bottom of the stone is clean and dry. With most of my opals I use super-glue gel to attach the dop sticks. (I have found that the gel version is much easier to use than the regular super-glue.) I use a few small drops of super-glue gel and wooden dop sticks. After polishing, to remove the opals from their dop sticks, I either let them sit in a carefully sealed container of acetone or soak them in warm water or just saw the sticks off close to the bottom of the opal (using a small saw - no larger than 6"). If I have placed them in either acetone or warm water, I apply light pressure to try to pop the opals off their dop sticks after they have spent about 30 minutes soaking. Usually at this point most come off and some do not. Sometimes I need to let them soak overnight. If the back of the opal is very porous, then I might use dop wax instead of super-glue.

I have also used green dop wax. If you use dop wax it is important to heat the opal gently before dopping it. Do not let it get too hot. I preheat the opal by resting it on the edge of the dop pot for no more than 30 seconds. To remove the opals from their dop sticks I would put them in an open bowl, which was at room temperature, and then place the bowl of opals in the freezer for about five to ten minutes. After removing the bowl from the freezer I would carefully place the blade of a knife along the line between the bottom of the stone and the dop wax and apply gentle pressure. Usually the stones pop off rather easily.


If you are using a machine that recirculates its water (i.e. the water is sprayed up onto the wheels, drips down into a pan, then is pumped from that pan and sprayed up onto the wheels again) you should change the water and clean the pans frequently when cutting opal. Chunks of ironstone and opal that were removed with the 80 and 220 grit wheels can splash onto the 280-grit wheel while you are using it and cause scratches. The same is true with the set of finer wheels. You could prepare several stones using the 80 and 220 grit wheels and then change the water. On the other side, you could cut several stones using the 600 grit wheel and then change the water before you move the stones on to the 1,200 grit wheel.

I have adapted my machine so that fresh water drips directly onto each wheel and drains out through the pans.

Once the glue has dried or the wax has cooled, look at your opal. Look for scratches and for the shape that the opal would naturally tend to be (which might not be the same as the shape you want it to be). I usually use the 280 grit wheel, with light pressure, to remove the serious scratches from the edges and the top of the opal, remembering to keep the stone moving to avoid flat spots and scratches. Stop frequently and wipe your stone clean to check your progress and make sure that the serious scratches are disappearing. To check my progress I use a lamp with a 100-watt bulb. Remember to look at the surface of the opal and not into the opal. I look for the reflection of the light bulb on the surface of the stone. This helps me to stay focused on the surface of the stone. Once the serious scratches have been removed I use the 600 wheel to remove the scratches caused by the 280 wheel and to expose more of the opal on the surface.

You should not be in a hurry. It takes a while to develop the feel for cutting boulder opal. Patience now will reduce your frustration later.

As I try to expose the opal I often find stubborn islands of ironstone on the surface and mountains of ironstone that rise up through the opal layer to the surface. This is the nature of boulder opal. Be careful about trying to get rid of some of those islands of ironstone. Some of them run deeper than you think. Probably some of them are mountains and not islands at all. Sometimes trying to get rid of those islands will severely distort the shape and the look of your stone. Be careful not to ruin your stone because of a few small spots of ironstone. These unique ironstone patterns can add personality to your stone.

At this point you need to make some important decisions about your opal. You need to look at the overall appearance of your stone, both the opal and the ironstone. Often the combination of the opal and ironstone on the surface can be quite beautiful. If you are satisfied with the appearance of your stone, then most of the tricky work is over and you can finish your stone. If you are not satisfied, or if you think that there is more treasure lurking beneath the ironstone, then you need to continue removing ironstone with the 600 grit wheel, remembering to keep the stone moving. The 600-grit wheel can still remove opal and create flat spots and scratches.

I am doing several things at once at this stage in cutting. I am removing scratches from earlier stages. I am removing ironstone from the surface. I am making the surface of the stone and the shape of the stone more pleasing to the eye. Sometimes as the surface is being worked on opal is revealed or destroyed or attractive patterns in the ironstone are exposed. Any of these things might influence you to adjust the overall shape of the stone. Look at your stone and see what it shows you. Look and see what shape it would naturally tend to be. You might need to go back to the 280 wheel to change the shape of your stone. Once you have removed the large scratches and are satisfied with the look of your stone move to the 1,200-grit wheel.

The 1,200-grit wheel will remove almost all the visible scratches from your opal. It will not do much to change the shape of your stone, but it can improve its appearance. Use moderate pressure, once again remembering to keep the stone moving. Make sure to cover the entire stone including the sides and areas of ironstone. The effects of the 1,200 grit wheel on your stone are not as immediately obvious as the effects of the 280 or 600 grit wheels. Using the 1,200-grit wheel effectively can make polishing a quick and simple process. Stop frequently and wipe your stone clean to check your progress and make sure that the scratches are disappearing. The 1,200-grit wheel is also useful to slightly round off those sharp edges that sometimes occur while you are cutting. Once the visible scratches have been removed move to the 3,000 grit wheel, or start to polish your stone if you do not have a 3,000-grit wheel.

If your machine has a 14,000-grit wheel I strongly suggest not using it to cut boulder opal. I have found that this wheel often introduces scratches. Many of the Australian miners and cutters I know also skip the use of this wheel.

I have found that using light to moderate pressure on the 3,000-grit wheel will brighten the stone and can greatly improve the finish on the ironstone areas of a boulder opal. It appears to me that the 3,000-grit wheel plays a very important role in achieving a great finish on your stone. If you do not have a 3,000 wheel, I highly recommend buying one.

Once again, stop frequently and wipe your stone clean to check your progress. When you are finished with the 3,000 wheel there should be absolutely no visible scratches. With many stones, after using the 3,000 wheel, you will be tempted by the beautiful finish to stop at this point. That is your decision. However, I recommend that you move on to your 8,000-grit wheel.


To polish, I use an 8,000 grit Nova wheel. If you are using an 8,000 grit wheel to polish, it is very important that you have first pre-polished your stone on a 3,000 grit wheel. I have found that I will not get as good of a finish if I skip the 3,000 wheel and go right from the 1,200 wheel to the 8,000 wheel.

I use light to moderate pressure on the 8,000 grit wheel. If the opal you are cutting is mostly matrix, then you will want to spend a slightly longer amount of time on this wheel than if you were cutting a mostly clean-faced opal. In general, I have found that not as much time is required on the 8,000 wheel as on most of the other wheels. If your opal has been properly prepared up to this point, the 8,000 wheel will give your opal a very nice finish rather quickly.

Using an 8,000 grit wheel properly will give your opal a great finish. Because there is no polishing compound to get stuck in gaps or porous areas, it gives a particularly good and trouble free finish on matrix opals or opals which have a large percentage of ironstone on the surface.

It is very important that the 3,000 and 8,000 grit wheels are properly "broken-in" before you use them for cutting opal. I usually will slowly cut 2 or 3 quartz (or agate or jasper or even hard ironstone) cabs on the wheels, making sure that I go from edge to edge on the wheels, before I use them to work on opal. It will take longer to break-in the 8,000 wheel than the 3,000 wheel.

If, after using the 8,000 grit wheel, you see scratches on your opal, it is probably due to one of these three reasons. (1) The most common reason probably would be you did not properly prepare the opal. In other words, the opal was cut too quickly. Not enough time was spent on each wheel making sure the scratches were completely removed from the previous wheel. The solution is to go back and re-cut the opal more carefully. (2) A very common reason for scratches is the 3,000 and 8,000 grit wheels were not properly broken-in. The surfaces of both of these wheels should look and feel clean. They should not feel "scratchy" to the touch. The solution is to gently, but firmly work a quartz cab across these wheels. (3) Another common source of scratches is contaminated water. If you are using recirculated water (water sprays up onto your wheels from the pans below) the solution is to change the water frequently and particularly between the 3,000 and 8,000 grit wheels. If you have fresh water dripping down onto each wheel sometimes water will drain along the inside of the housing from above the more coarse wheels to the finer wheels. The easiest solution is to elevate the end of the machine which has the finer wheels a few millimeters. This will cause the water to drain toward the more coarse wheels.

To polish, I have also used tin oxide, for mostly clean-faced stones, or Linde-A, for matrix stones, on a foam-backed rough leather pad. I make sure that the pad is clean. I lightly wet it with water from a spray bottle. With the machine running and the pad spinning, I apply a well-stirred mixture of tin oxide and water onto the damp pad with a brush. The tin oxide and water mixture is thin enough so that it does not clump, but thick enough so that when it is stirred it feels as if it is thicker than plain water. It has the consistency of slightly thin gravy. I next select a dopped opal to be polished and either gently touch it to the polishing pad so that it picks up some polish or just touch the opal with the brush used to apply the polish and water mixture.

The opal should now have some polish on it, not necessarily covering the entire stone. Using light to moderate pressure I start polishing the opal on the pad. Remember to keep the opal moving by rotating the dop stick between your fingers and by changing the angle between the opal and the pad. Check your progress frequently by gently wiping the surface of the opal clean with tissue. Remember to be looking for scratches on the surface of the opal. Do not be seduced by the beauty of the opal and look down into the color. Keep your eyes on your polishing job on the surface of the stone. Remember to polish the edges of the stone as well. Keep a firm hold on your stone and do not apply too much pressure. Do not let the opal get too hot. Sometimes the rough leather pad can grab your stone and yank it out of your hands, particularly as the pad dries out or if your stone has an uneven surface or sharp edges.

It takes a bit of practice to determine when you are finished polishing an opal. You want to remove all the scratches as well as make the opal as bright as possible.

It is important to keep the pad wet and reapply the polish and water mixture when needed. If the pad gets too dry it can cause the opal to overheat or scratch it.

If your opal has much ironstone on the surface or if the ironstone on the surface is particularly porous, be careful. Often the leather pad will catch on the ironstone and either you will feel a tug on the stone or the pad will tear out small pieces of the ironstone that could damage or scratch your opal.

If you are polishing a matrix opal follow a similar procedure. Often the opal in such stones will polish normally, but the ironstone matrix will be dull. There are a few ways to deal with this situation depending on the type of ironstone. One technique is to use Linde-A in a manner as described above. Another useful technique is to polish the opal as already described and then let the pad dry slightly. Apply more pressure, while you keep the stone moving, so that you can feel the stone get warm. Remember you are polishing the matrix areas not the opal areas of your stone. This requires careful judgment. Gently heating the stone on the polishing pad can improve the finish of the matrix, but, if it gets too hot, it can damage the opal. This technique can be dangerous and requires a bit of practice, but the results can be worth it.

Once polishing is finished remove the stone from its dop stick according to the instructions found in the section on dopping.

It seems that every couple of years I hear about some new miracle polish that does a great job on boulder opal and just about everything else. I have become rather skeptical about such claims.


Once the stone has been removed from its dop stick I use light to medium pressure on the 220 or 280 grit wheel (or the 600 grit wheel for more precious, more fragile, or smaller stones) to remove any remnant of the dopping material. I keep the stone flat against the wheel, but still rotate it slightly so that there will not be any deep scratches on the back. At this point I decide what I would like the final thickness of the back of the stone to be. In addition to removing any dopping material, I usually need to remove a little ironstone to reduce the thickness of the back of the stone. Once I have reached the appropriate thickness I usually use the 220 or 280 (or 600, as above) grit wheel to bevel the back edge of the stone which will reduce chipping when setting the stone.

Depending on the appearance of the back and your taste, you can either consider your opal finished at this point, or you can achieve a more finished appearance by working the back of your opal through the remaining series of finer wheels. I usually work the backs of my stones through the 1,200 or 3,000 grit wheels. Sometimes I polish the backs.

Remember, cutting stones, particularly cutting boulder opal and Koroit opal is a combination of art and science, aesthetics along with technical ability.  Cutting stones is a learning process where the education never ends.  I have described the way I cut stones now.  I am always looking for new and improved methods and techniques.  Good luck as develop your process.