SANDPAPER METHOD

Cheapest but Slowest (still works though)

The most simple method to get started with is the use of sandpaper. This method also gives you an unrivaled feel for learning the response of the stone to the abrasive which can help you a great deal even when upgrading equipment. I have covered this process in video form on the YouTube channel in quite a lot of detail if you prefer to learn via video (bottom of section).

Precautions

This method is quite harmless but there are some things to be aware of.
1/ Use water at all stages as opal dust is silica and can result in silicosis if breathed in.
2/ Use a mask if you find that you are producing any dust.
3/ Be cautious of grinding away your skin on the sandpaper. Dopping the stone can help you avoid this.
4/ Take a break every now and then. This is a very tough process on your hands since there are no power tools involved and you can get some of the craziest cramps in your hands if you push too hard (trust me).

Step One: Rubbing

Starting out you need to know what you've got.
Remove "potch" (no play of colour common opal) around the opal bar to expose it fully following any small glimpse you have with a low grit wet sandpaper (80-120 is pretty good to start depending on the opal).
Once you've got that done you should be able to determine which side of the bar you want to fully reveal but don't go to far with the low grit! Stop just before you hit the colour or the deep scratches will be hard to remove without losing too much colour.

Step Two: Shaping

Decide on the shape you want your final stone to be and continue with the low grit wet sandpaper until you are pretty happy with the shape. You should maximise the size of the stone whilst also removing any inclusions such as sand, cracks and/or potch. This is a great time to use a stencil for the perfect shape and it also helps to see the inclusions you will be removing.

Step Three: Pre-Polishing

Now that you have a rough shape you can begin the final fine tuning of your shape and begin to get to a pre-polish stage.
Slowly step up grits from your starting coarse grit until you reach at least 3000, though I prefer a 5000 grit wet sandpaper made for glass and automotive applications. Between each grit wash your stone with water and dry it! The drying part is important as you can see the progress of your hard work and you should see the scratches getting smaller and smaller. Over time you will learn what each grit's scratches look like.
By the time you get to your highest grit the opal face should be fully revealed and shape finalised. The scratches should also be very hard to see and the surface should be foggy rather than scratched.

Step Four: Polishing

My fave part!!! Here you just need your choice of polishing compound which can be a metal oxide such as cerium, tin, aluminium oxide or a range of diamond pastes.

For most opal I recommend cerium oxide though depending on the opal other options can be slightly more effective in terms of efficiency and outcome. I have a video detailing this on the channel (https://youtu.be/IooWs2tndmo).

To use the cerium oxide powder you just need to mix up a slurry then apply this to a leather or dense felt pad.

Rub your stone in the slurry adding water if it begins to dry.

Once the opal is beginning to get a good shine to it keep rubbing but let the pad dry.

By the time the pad dries out you should be able to give your stone a wash and be left with a beautiful glossy surface.