Part 1: Selecting & preparing rough, exposing opal.

Part 2: Dopping, Cutting, & Finishing

Australian (Queensland) boulder opal is a captivating gemstone. As the popularity of boulder opal has grown, so has the interest in cutting it. It is true that some boulder opals can be quite challenging to cut, but if you are able to cut other gemstones you will be able to put your skills to good use here.

Australian boulder opal rough Dop sticks Dop wax or strong glue Trim saw with thin blade (rim thickness approx. 0.02") Diamond grinding wheels, 80-, 220-, 600-, 1200-, and 3000-grits Cutting equipment Rough leather polishing disk Tin oxide polish Lamp with 100-watt bulb Safety glasses Dust mask Old work clothes For information on supplies, please see the Annual Buyers' Directory.

STEP 1. Selecting Rough
Buying boulder opal has its own special concerns. Like any rough, the more you know, the better. I do not make any promises, but I will make a few suggestions.

Ask dealers for suggestions for finding inexpensive rough that will be relatively easy to cut. Make sure that the ironstone around the opal is fairly sturdy. Beware of the deals that sound too good to be true. Buy fairly low grade material to start with. You might not end up with a gorgeous stone right away, but you will improve your technical skill enough that, maybe, you will be able to cut that beautiful stone in that next parcel of rough.

STEP 2.
Look for rough with a layer of opal that is as flat and consistently shaped as possible. Look at the grain and pattern of ironstone to give you clues about the direction in which the seam will run. When opal occurs in ironstone that shows fairly flat, straight parallel lines in the matrix, there is a good chance that the opal will follow these lines. If the ironstone displays wavy lines, the opal will probably be rather wavy.

Look for cracks. Avoid rough that has been soaking in oil or glycerin, both of which hide cracks. If the rough is wet, let it dry while you are talking to the dealer, then look for flaws. Some pieces of rough may be partially faced; that is, 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. Avoid faced material that has deep gashes in the opal layer.

STEP 3. Preparing the rough
Put on old clothes. You'll be splashing around quite a bit of brown mud, which stains. If necessary, saw away the excess ironstone so that you can manage the remaining piece of rough with the trim saw.

Look at whatever line of opal is visible and at the pattern in the grain of the ironstone. Try to estimate the direction in which seam will run. Using a combination of the trim saw and an 80- or 220-grit wheel, expose the edges of the line of color. Try not to get too close to the line of opal. The seam of opal can make some unexpected turns, which you can ruin with the saw blade.

STEP 4.
Go back and forth between the wheels, cleaning off the stone often, and looking for signs of opal. As soon as you get one, stop and turn the rough to find other edges of the opal. Try to make sure the 80-grit wheel does not touch the opal - it can be very damaging. When you have found the edges, look carefully at the shape of the line and the nature of the ironstone on either side of the line to determine how to orient the stone.

STEP 5. Orienting the opal.
Decide which side is going to become the top. Base your decision on the shape of the seam of the opal, and the coloring, 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) and the bottom of the stone is the concave (inward curving) side.

Ideally, the material that is immediately below the line of opal should be as dark as possible. This dark background will make the colors of the opal stand out. Occasionally, there will be a thin layer of black potch between the line of opal and 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 black boulder opal, and can be quite brilliant.

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

STEP 6. Exposing the opal.
I start with the 80- or 220-grit wheel to remove the ironstone that is covering the top of the opal. I only use the 80-grit wheel if the ironstone layer is more than approximately 4mm thick. When using the 80-grit wheel, be careful and use light pressure. It can cause chipping or make deep grooves.

When removing the ironstone layer, be careful and use water. Repeatedly stop and wipe off the surface of the stone to check for exposed opal. The seam of opal can take surprising turns or bulge in unexpected areas.

STEP 7.
As you get closer to where you think the opal is, use the 220- and 280-grit wheel. Try not to come into contact with the opal when using the 220-grit wheel, as it can quicly remove quite a bit of colorful material or introduce serious gouges. Use increasingly lighter pressure with the 220-grit wheel as you get closer to the opal.

Continue to remove ironstone until you just barely expose opal in one area of the surface, or at least until you are very close to the opal layer. Sometimes, just before you reach the opal, the coloring of the ironstone will change.

STEP 8.
Switch to the 600-grit wheel to try to expose some of the opal. At this stage you are not trying to remove all of the ironstone from the surface. I usually go back and forth between the 600-grit wheel, using medium pressure, and the 280-grit wheel, using light pressure. Once you determine how the layer of opal runs, stop working on the top.

When you have a good idea of the shape of the opal, use light pressure on the 220-grit wheel or medium pressure on the 280-grit wheel to shape or 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.

Next time we will dop, cut, and polish the opal.

Eugene McDevitt is a Florida-based importer, cutter, and wholesaler of Australian boulder opal and other stones. He can be reached by e-mail at gene@koroit.com, on the web at www.koroit.com, or phoned at 727-742-5471. Mailing address: P.O. Box 20288, St Petersburg, FL 33742.

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