Cutting Opal is really about grinding away the colourless opal or potch and shaping your stone to an appealing shape while revealing the best colour on the prominant face of the stone.
As explained elsewhere great care should be taken to study the stone before any grinding is carried out. Look at the stone from all angles under a good incandescent light or sunlight and work out which face has the brightest colour. Study the stone with and without using your 10 magnification loupe so you can see any surface irregularities , sand or cracks which will have to be removed before polishing.The face of the stone with the brightest colour should be the top of your finished stone. Mark this with texta colour and then proceed to dop your stone with the wax being on the opposite side to the selected top. (See the post on Dopping Opal on this page) You may need to grind away any sand or loose material on the back of the stone to ensure the wax will get a grip.
This first stage of grinding could be carried out on a coarse diamond (400-600) or a finer carborundum wheel (800). Do not use coarser carborundum wheels as they can shatter the edges of the opal and can ruin the stone. Another tip is to apply only light pressure to the wheel and keep the stone moving at all times. I cannot stress the importance of this enough. It is better to grind the stone slowly than wreck it by too much pressure and being impatient. Also remember to grind on the bottom half of the wheel with the wheel turning toward you and keep plenty of water on the wheel. Do not breathe any of the dust as it can be very dangerous.
Now grind around the edge of the stone at right angles to the top until you can see traces of colour all around the stone. Do not hold the edge of the stone still against the grinding wheel or you will grind a flat section which will look ugly and result in having to grind away a lot of the good colour in order to remove it. Holding on to the dop twirl it in your fingers slowly as you hold the stone against the wheel. Do not apply too much pressure and you will soon have a rounded edge to the stone. At this stage don’t worry about the shape of the stone. You are just grinding away the opal with little or no colour which will mar the finished stone if you leave it.
Once you can see colour all around the edge of the stone have a good look at the shape. You now have to decide what shape to make the stone. It would be very unusual to complete the first stage of grinding and have a good shape to the stone. The shape will usually be irregular and require further grinding to produce a shape that appeals to you.
If the shape is almost round DO NOT try and make it perfectly round without using a faceting machine. No matter how hard you try and how careful you are it will always look like you have tried and failed. It is far better to turn it into an oval or a more irregular shape. I always prefer to also avoid negative curves especially while learning. They are difficult to cut and even more difficult to form the top of the stone into a pleasing shape.
Still working around the edge, grind away those sections that are not required until you have a pleasing shape. This usually means no sharp points or flt sides. Try and have a free flowing smooth edge at this stage and the stone will look really good when finished. Once you are satisfied with the shape you are ready to move onto shaping the top of the stone. I amm assuming you have some experience with cutting a cabechon already. If not it is better to practice on some potch or other material anot learn on precious opal.
Before actually grinding any of the top of the stone examine it carefully. You need to study how the colour is spread through the stone. If it is evenly spread you can cut a high domed top. If the colour is a narrow bar you will have to cut a flat domed top. If the colour is wavy you will need a wavy top. Cutting opal is all about getting the best colour from your stone. It is no good having a lovely shaped stone with the best colour cut away.
Having studied the stone you now need to shape the top to reveal the beauty from within the stone. Using a very light touch and broad strokes grind off a thin coating of the top to more clearly be able to see the colour. Once you are happy that you can see the colour clearly hold the dop stick at an angle of around 30 degrees and grind away all around the top edge of the stone making an even depth face all around the stone. Increase this angle to about 45 degrees and repeat the process. The amount you take off will depend on the amount of doming of the top of the stone. Repeat this process at differing angles until your dop is about 75 degrees to the wheel . You will now have a whole series of little flat faces cut all around the stone. All of this stage should be carried out on the finest grinding wheels you have.
The next stage is to move the top face of your stone from one edge to the other in one sweeping movement just lightly touching the wheel. Do this many times turning the dop stick slightly in your fingers each time. This will remove the ridges created by the first stage of the grinding process.
As you get confident try twirling the dop stick in your fingers at all different angles while letting the stone very lightly touch the wheel. This will further remove any ridges and flat spots and assist in the sanding and polishing processes.
Remember to stop and frequently inspect the stone using your loupe. Look for any irregularities such as sand spots, cloudy areas where the colour is not clear, cracks or other blemishes on or in the stone.You may have to decide to leave a blemish in the stone if the alternative is to cut away a beautiful piece of colour but if possible, it is better to remove that blemish as it will take away from the ultimate beauty of the stone. In my opinion it is better to have a smaller stone which is blemish free rather than a bigger stone with many blemishes however the decision is yours. If you want to remove any sand spots it is better to do it with the grinding wheel rather than at the sanding phase as it is much quicker. Do remember not to hold the sand spot or blemish up against the wheel without moving the stone. If you do you will get a flat spot which means even more grinding away of the colour to get a smooth regular domed surface.
The aim of grinding the surface is to end up with a domed top. Do not try to make the top dead flat. It will take many hours to sand and polish a flat top compared to minutes for a domed top.
Once you are satisfied with the shape of the stone you can un-dop it and then re-dop the stone with the wax on the top face. In many cases I would go on wth the sanding and even polishing while it is dopped at this stage but as these techniques are in a separate post I will proceed with the rest of the cutting methods.
To un-dop the stone carefully heat the wax close to the stone using your spirit lamp until the wax is pliable but not melting and peel it off the stone. If some wax sticks drop the sone into some methylated spiirits and leave for 10 minutes or so and it can easily be removed. DO NOT place the waxed stone in a freezer as you can do with some stones because the water content in opals can cause them to shatter in extreme heat or cold.
To be continued