Items Needed for Cutting and Polishing Opal

7 June 2010 by Johno

Categories: Opal Cutting | Australian Opal

The cutting and polishing of opal will require some machinery in order to shape and polish the stones. There are many different types of machinery available and it can be very expensive. If you are just starting to cut opal or any other gemstone, you can do so without spending a lot of money. This post will describe probably the minimum amount of equipment you will need to start with. As you get more confident you can then experiment and upgrade to better quality equipment. With the equipment I will describe you will be able to cut and polish beautiful stones: you may just have to be a bit more careful than if you had the more upmarket equipment.

The equipment you will need is as follows:

1. Some form of grinding wheel assembly. Diamond impregnated wheels would be the best but they are expensive. The much cheaper carborundum wheels can be used if you are careful.

2. Sanding discs of various grades. Wet and dry sanding paper.

3.A felt polishing disc.

4. A spirit burner.

5.Dopping sticks.

6. Cerium oxide or tin oxide polishing powder.

7 A water supply.

8. Good light.

9. A loupe with 10 times magnification.

I will now discuss each of these in some detail.

1. Grinding wheels

If you have a cheap grinding wheel assembly this will be quite adequate.

When using carborudum wheels make sure they are kept faced. That is, make sure the cutting surface is flat and not concave.

The wheel needs to be spinning towards you and always cut on the section on the lower section of the wheel. If you try to cut on the upper section the stone will be very hard to control and could cause damage to you and the stone.

With opal it is better to use the finer grade of wheel rather than the coarser ones. Opal can be quite brittle and the coarse wheels can cause shattering on the surface. I would use 800 grit and 1200 grit wheels if you can get them.

Dimond impregnated steel wheels are better because they feel much smoother and will cause less damage.Also the carborundum wheels can very quickly grind a nasty hole in your finger if you are not very careful.

When cutting opal you need to keep the stone wet at all times for 2 reasons.

(a) Excessive heat could build up in the stone and cause it to shatter.

(b) The dust from the grinding process is a fine silica and can be very dangerous to your health if breathed in.

To prevent any problems you can dip the opal in a tub of water frequently while cutting or rig up a plastic bottle with a tap and a plastic tube which lets water drip continually on the wheel while you are cutting. This means you will get wet and will need a waterproof apron.

2. Sanding discs

After cutting an opal you need to get rid of all of the small ridges created in the cutting process. To do this you need to make a sanding disc. You can make this yourself by cutting a 150 mm diameter circular piece of 5 ply wood or equivalent thickness of other wood. Drill a hole in the middle so you can attach it to the grinding wheel assembly after having removed one of the grinding wheels.

You now need to glue securely a thin piece of rubber sheet onto the flat face of this disc. This is needed when sanding to give some flex in the surface to take out the ridges left from the grinding wheel.

When the rubber is set, you will need to glue on discs of wet and dry sanding paper of the appropriate grade which we will discuss later. I use UHU glue stick for this as it allows the worn sanding paper to be easily removed without permanently sticking to the rubber sheet.

This one disc can be used for all grades of paper by removing the worn disc and replacing with another cut from a standard sheet of wet and dry sanding paper.

3. Felt polishing pad

Using a similar wooden disc as used for the sanding process, glue on a disc of dense felt. You can get these from lapidary shops quite cheaply.This is used with cerium or tin oxide and water to give a very high polish after the sanding process.

4. Spirit burner

This is a small glass bottle with a lid with a small hole in the top through which a wick of thin cotton rope is pushed. The jar is filled with methylated spirits. Thsi spirit then saturates the wick and you can safely light the wick as long as the top is sealed. This gives you a heating source to warm the stone when dopping and to melt the dopping wax.

5. Dopping sticks

It is very difficult to cut any opal and especially small stones by holding them in your hand. For this reason it is essential to firmly fix the stone to apiece of wood called a dopping stick.

You can make these by cutting 150 mm lengths of 7mm diameter dowel. This will be ideal for most of your stones. For very small stones you can cut similar lengths of kebab skewers or even use nails although i prefer the feel of the wood.

6. Dopping wax

This is a special wax use to firmly stick the opal onto the end of the dopping stick so that you have much better control of the cutting and polishing processes. This wax can be purchased in small sticks from the lapidary shop.

Some people prefer to use various sorts of glues or 2 part resins to set the sones but I am too impatient to wait about 24 hours for them to set properly.

There can be some problems using the wax and I will discuss them further in the cutting and polishing section.

7. Wet and dry sanding paper

As mentioned earlier you can cut discs from standard sized sheets of wet and dry sanding paper. You need this type of paper because the opal needs to be kept wet at all times during all of the cutting, sanding and polishing stages.

I use 800 grit paper for the initial sanding and then 1200 grit for the pre-polishing. Worn 1200 paper will give an excellent finish and prepare the stone for the final polishing process.

I use UHU glue to glue these sanding discs to the rubberised disc as it can be removed easily when the sanding disc is worn.

8. Cerium or tin oxide powders

Either of these powders will give a very high polish to opal as long as EVERY scratch that is visible using a 10 times magnifying loupe is removed and all flats are also removed at the sanding stage.

The best method of using the powder is to add two flat teaspoons of the powder to one of those cheap spray bottles and mix with water. You then spray this water onto the felt pad until it is saturated. A lot will flick off when the disc rotates but you only need a small amount to polish the stone. Often people make a big mistake in using large amounts of the powder but this is not needed.

Many people prefer to use diamond pastes of various grades up to 100,000 microns but these are not essential as these oxides will give a very satisfactory polish in quick time if you prepare the stone carefully.

9. Good light

Do not try and cut stone under fluorescent or dim lighting.

An incandescent globe of 60-80 watts is very good and will allow you to see the colours in the stone much better than the fluorescent globes. Remember that cutting opal is all about getting the best colour out of the stone and you can only do that if you can see it clearly.

10. Magnifing loupe

This is probably the most important piece of equipment you will need if you are to cut and polish a beautiful stone.

You will not get a quality finish unless you remove every faint scratch and every flat surface on the face of the stone at the sanding stage and you won't be able to do this unless you can see them clearly.

The naked eye is not good enough even if you have 20:20 vision.

You will need a 10 times magnifying loupe to effectively see these scratches and flats.

If you have all of the above, and of course an electricity supply, and use care and patience you will be able to transform a rough piece of opal into a beatiful, highly polished gemstone which you will be proud of.

To be continued...

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