If you have any thin pieces of opal with nice colour but believe it is too thin to cut a cabechon then think about making a doublet.
A doublet is basically a fairly thin layer of usually opal glued to a thicker layer of opal potch or some other material to make a stronger stone. A cabechon is then cut from the prepared material. Don't be afraid to have a go at making doublets. It is not too hard and you don't need a lot of equipment. The equipment you will need, in addition to opal cutting equipment is as follows:
(a) A small sheet of flat glass. A piece of window or mirror will do quite well.
(b) A small amount of 400 and 800 grit size carborundum powder.
(c) Araldite or similar epoxy glue.
(d) Some lamp black powder or similar fine black powder if using crystal opal. ( I will explain why later)
Thats all the extra equipment you will need to produce a really nice and valuable gemstone.
The first step is to carefully examine the thin layer of opal. It will need to be thick enough to form a shallow cabechon top after the base has been made flat. You want the brightest side to face upwards. Having selected this side than you need to make the back side perfectly flat and to remove any pits or pieces of sand. To do this carefully clean your your piece of glass and place a small quantity (Quarter of a teaspoon will do) of the coarser carborundum powder on the glass and mix in a little water to make a wet mix. Then place the back of the opal slice onto this mix and with your finger on the top of the stone, rub the opal on the glass in a rotary motion. You need to keep doing this until the back of the stone is very flat and free of imperfections. If you leave any imperfections they will be readily visible when you finish the stone and you will be disappointed.You can tell this when there is an even dull sheen across the back of the opal.
Carefully wash the opal and then repeat the process using the finer powder. This will not take very long as the opal is already flat from the coarse paste. You do not have to polish this face as the glue will fill in any slight imperfections.
You now need to select some material for the doublet backing. The material used is not critical although purists like to use opal potch as the combined stone is then technically all opal. Opal potch, particularly black opal potch is not all that easy to obtain so you can use any other material. If you are using crystal opal for the top of the stone it is advisable to use a black backing as this brings out the colours of the crystal opal much more than a paler backing. If you are using white opal for the top then a backing of a material of roughly the same colour as the opal is a better solution.
The backing needs to be about twice the thickness of the opal top . If the opal top is very thin you may need to glue on a cap of transparent material in order to cut the top and this then becomes a triplet (three layers) I will talk more about that in another post. I have used opal potch, black glass and obsidian and all have been ok. I did use plastic from an old LP record once but found when cleaning the finished stone with with methylated spirits later, the backing started to dissolve so I definately stick with stone or glass for the backing.
Having selected your backing you now need to saw or grind it to a thickness of about twice that of the opal top and to a size marginally bigger than the opal top. Now select one side as the top side which will ultimately be glued to the opal top and proceed as you did for the opal top using the coarse and fine carborundum on the glass until that side is very flat and even. Only do this to one side. Don't worry about the back.
You will now have a flat backed opal top and a flat topped backing piece. It is definately easier if the backing is slightly larger than the top as you will find out when glueing the pieces together.
The next step is to glue these together. Here you have to be very careful to make sure the two flat pieces are very clean. It is best to wash them with water and then place them, flat side up, under a heat source such as a light globe and allow them to dry before glueing. Do not wipe them with rag or tissues as small pieces of lint can collect on the surface and when glued they can be seen easily and can ruin all your good work.
When the two pieces are dry, place them flat side up and then mix the epoxy. If I am using crystal opal with a black potch backing, I mix the araldite very slowly and carefully with a small quantity of lamp black to provide a dense black glue.The idea is minimise the formation of any bubbles in the glue. Then cover each flat face with the mixture ensuring your fingers only touch the edge of the stones. Press the two pieces togethher and apply pressure on the opal top and rotate it against the base to get rid of any air bubbles. This is very important as an air bubble is easily seen and can ruin the stone.
I believe you can make lamp black by holding a candle very close to a piece of glass. A black coating (soot) forms on the surface of the glass and can be scraped of and used. You can also buy it at lapidary suppliers or on line.
If you are joining an opal top that is not transparent then of couse you do not have to add the lamp black.
I have attached a sketch of the cross section of the finished doublet. The dashed section around the edge is the original two pieces glued together. The solid line shows the finished shape you are aiming for. There are a couple of important things to note. The first is that the edge of the araldite and lamp black layer, when finished, needs to be slightly inside the edge of the stone. This is so you cannot see it when you look from the top. If you can see the araldite layer from the top the stone will look unsightly. Second it is better to cut the bottom of the stone with a slight dome especially if you are going to polish the bottom of the stone. It is not necessary to polish the bottom especially if it is going to be set in jewellery but I always do as it looks better when people view the stone unset.
The first step in the cutting is to grind all around the edge of the stone until you can see a thin straight line of the araldite mix right around the stone. This is important. Don't worry about the shape of the stone at this stage. Once you have ground the opal and base so that you have this even straight line of araldite then look at the shape of the stone from the top. It will probably be irregular and unshapely. It is now time to get it to the shape you want by grinding away at the edges. You can mark the shape on the back of the stone if you wish or just use your eye to get a nice shape that you like.
One you have the desired the shape it is time to cab the stone. I am assuming you have cut cabechons before. If not, it is best to practice on some potch just to get used to cutting the required shape. I have a post on cutting opal that may help.
Cutting doublets does require a bit more care than cutting solid opal as the stone has to be even across the top and the araldite layer needs to be horizontal when viewed from the edge.
If you follow these guidelines and those in the opal cutting post I am sure you will end up with a lovely stone.
Hope this helps. Please let me know if you have any problems.