Cutting Solid Opals
I have been cutting opal since I was thirteen. I have cut Mintabie opal, Lightning Ridge opal, Andamooka opal, Coober Pedy opal and Boulder opal from many Queensland opal mines.
This article is about the basic procedure I use to cut opal to produce the best results for the cut solid opals that we sell in our shop and our website. (Boulder opal and opal doublets will be handled in separate articles)
Firstly a few hints. Attention to detail. Don’t move from one step to the next unless you are totally satisfied with the result you have achieved at each stage.
Opals ready for Dopping
Don’t be afraid to try a new technique. Most opal cutters have their own method and it is a matter of finding out what works well for you.
A good idea is to watch as many good opal cutters that you can. You will learn a little from each.
Learn to cut cabochons from cheaper gemstones such as agate before you move on and try and cut opal. Practice cutting material that is around the same hardness as opal such as obsidian or the many common opal and opalites that are available at some lapidary outlets. I believe it is better to make mistakes on material that is not too valuable. Once you are proficient at cutting good cabochons move on to precious opal.
I tend to cut opal solids in batches, doing each step in lots of 20 or so opals.
Opal Cutting Machinery
I use a Diamond Pacific cabochon cutting machine fitted with two 8 inch diamond impregnated cutting wheels (80 and 220 grit) and four diamond 8 inch diamond impregnated sanding wheels (280, 600, 1200 and a new spare of one of these that I am wearing in). The end of the machine has a flat disc that has a rubber pad attached to it. On top of this I have glued a ‘polypad’ that I impregnate with 100,000 diamond paste to polish the backs of my opals.
I have a seperate horizontal lap that has a fine quality 8″ leather disc overlaying a thin soft rubber pad. I use this for polishing the tops of my opal. I also have a small saw fitted with a very thin 4” diamond blade.
Step 1. First Shaping of the Opal
Generally using the 220 cutting wheel I remove the outer layers of the opal so that I can see where the colour bars lie. (Occasionally I will use the 80 grit wheel if there is a lot of potch to remove or the opal is low quality.)
This stage reveals any sand spots or flaws that will determine what my next step is. It is common to cut several stones out of a single piece opal depending on where the colour lies, the thickness of the opal and any imperfections that become evident. I use a thin permanent marker pen to draw saw lines that I follow.
A mistake many first time opal cutters make is to cut the biggest stone they can out of each piece of opal rough. Sometimes more value can be achieved by cutting the opal rough into several stones that are individually more appealing than one large stone. I believe the right idea is to cut the best stones out of each piece. If the opal is of fine quality I will cut stones often that are quite small (eg 4mm and 6x4mm).
Smaller patterned opal often cuts more valuable smaller stones per carat then if the same opal is cut into larger stones. Often rough opal with rolling flashes can be cut into incredible gems by positioning the flashes correctly when cutting. Some times this means sacrificing size but the value will be more. The most enjoyable part of opal cutting is making these decisions and achieving the result you have imagined.
Sometimes the colour pattern will vary within the opal rough so an artistic decision has to be made so that each opal is as good and as balanced as possible.
Often two or more totally different looking stones will be cut from the same piece of opal rough. Sometimes the top of one will be the bottom of its neighbor.
Cracks cannot be tolerated and are either ground out or sliced along.
If the opal becomes too thin to cut a viable solid I will set it aside for cutting into an opal doublet.
Often there are several colour bars within a piece of rough opal. A decision has to be made as to which of these bars will be the brightest or will have the most interesting pattern. I then grind down close to the colour bar that I want on top being careful to not get too close as the final cutting is best done on the sanding wheel.
I roughly preform the stone to a resemblance of its final shape. I flatten the bottom of the stone ready for dopping.
Step 2. Dopping the Opal
Dopping is when you ‘cement’ the stone onto a stick that you hold for the rest of the cutting process.
It is most important that the preformed opal sits properly aligned on the dopping stick. If it is not it is very difficult to cut a well balanced opal.
Every cutter has his favourite sticks of preferred length and thickness. I use bullet head nails of 7cm in length and of 3 different thicknesses depending on the size of the opal preform. These are cheap, and the uniform size means I get used to the feel of them. They fit in my palm and can be turned easily between my fingers. Feel in cutting an opal is important as it is a repetitive process and I believe it plays a major part in the process.
I use normal green dopping wax available at most lapidary suppliers.
I have a small saucepan with a thick base that I heat the wax in. To heat the wax I use a hot plate (electric) with a thick piece (about 1cm) of steel placed on top. The reason for all this thickness is so that once the wax is melting I can set the heat on the lowest setting and the wax stays at a perfect temperature (It needs to be liquid but just enough to hold onto the nail when you put it in. Too hot and the wax is too runny and wont hold the opals)
The plate of steel is about 200mm wider than the hot plate. On this I place the opals bottom up to receive the nails with wax on the heads. I also place the nails on this to warm to a temperature that is still easy to hold without being uncomfortable. I place the opals where they heat up to quite a hot temperature but not too hot that you couldn’t touch them.
The technique (once the wax is melted and the opal hot) is to place the head of the nail in the wax, lift it out and place it directly on the opal. I Lick between my finger and thumb, lift the opal up and make a cone shape under the opal. Don’t forget to wet your fingers otherwise wax burn is very painful as it sticks. Try not to get wax past the extremities of the opal preform.
Step 3. Final Shaping of the Opal
Now that the opal is on a dop stick and I can hold the opal better I go to the 220 grit cutting wheel and looking directly down on top of the opal and complete the final shape as carefully and correctly as I can. Minor adjustments can be done at the sanding stage but the better the shape now the easier it is later.
I try to get a right angle of about 10% of the final height of the opal around the base and then gently curve the rest of the opal. Moving on to the top of the opal I decide on the curvature of the surface. The amount of curvature is often determined by the thickness of the opal colour bar. I generally do a series of grinds around the opal ending up with around a 10 degree curvature on the top.
When I am happy that I have as near as perfect shape and I am happy that I have the best possible play of colour as close as I am prepared to go without actually going into it I move on to the 280 Sanding wheel
Step 4. Sanding the Opal
Before sanding the opal I check the shape is as perfect in all aspects that I can get.
The first stage of sanding is very quick. I simply remove all the marks from the cutting stage with the 280 sanding wheel and make any minor adjustments to the shape.
Almost as quick is the 600 stage. This only removes the coarser marks made by the previous sander.
By far the most time consuming part of my opal cutting is the final sanding, done on the 1200 sanding wheel. I do the edges first, holding the nail so that the opal edge is running at 90 degrees to the surface of the wheel. Looking down on top of the opal I check that my shape is perfect.
At this stage I use quite firm pressure and keep the opal moving at all times. I rotate the opal constantly and then move it up and down. I also put the main face of the opal on the sander and with firm rotating pressure work it until it is perfectly free of any sanding scratches. I dry the stone often and look at the reflection of the light on the surface of the opal. Any coarse sanding marks are then obvious and all need to be removed. The stone needs to be perfect in all aspects for this stage to be successfully completed.
Step 5. Polishing the Opal
This is the easiest part of the opal cutting process. If all the previous stages have been done correctly is simply a matter of moving the entire surface of the opal over the polishing pad until the opal gem is completely polished.
I have a spray jar that I keep spraying water onto the polishing pad.
My preferred polish is Tin Oxide. I mix this to a slurry with about 70% water and 30% methylated spirits. The mixture I keep in a sealed plastic jar with a thin paint brush. I brush the polishing pad with this mixture about every ten stones. I keep the polishing pad wet by consistant spraying with a water spray jar. The polishing pad is high quality leather on a softish thin rubber backing pad.
To remove the finished opal simply place in the freezing compartment of the refrigerator and it will drop off.
Step 6. The Back of the Opal
Sometimes I curve the back of the opal (Particularly if the opal is cut from a Lightning Ridge black opal nobby). If I am curving the opal I re-dop it and repeat the cutting process.
Most opal solids I cut have a flat back and I am used to holding the opal on the end of my finger and flattening the back on the 240 sander. I than follow the procedure through.
The final step is a small bezel put on the base of the opal that I also do by holding the opal between my fingers and turning it. I do this on the 600 sanding wheel